Fix compiler warning during building/testing.

[tnull]

This PR fixes the following warnings I got when running cargo build/cargo test:

warning: unused return value of `transmute_copy` that must be used
   --> lightning-invoice/src/lib.rs:120:11
    |
120 |     unsafe { core::mem::transmute_copy::<SystemTime, [u8; 16]>(&SystemTime::UNIX_EPOCH); }
    |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: `#[warn(unused_must_use)]` on by default

...

warning: lint `broken_intra_doc_links` has been renamed to `rustdoc::broken_intra_doc_links`
  --> /Users/ero/workspace/rust-lightning/lightning/src/lib.rs:23:9
   |
23 | #![deny(broken_intra_doc_links)]
   |         ^^^^^^^^^^^^^^^^^^^^^^ help: use the new name: `rustdoc::broken_intra_doc_links`
   |
   = note: `#[warn(renamed_and_removed_lints)]` on by default

...

warning: lint `broken_intra_doc_links` has been renamed to `rustdoc::broken_intra_doc_links`
 --> /Users/ero/workspace/rust-lightning/lightning-background-processor/src/lib.rs:5:9
  |
5 | #![deny(broken_intra_doc_links)]
  |         ^^^^^^^^^^^^^^^^^^^^^^ help: use the new name: `rustdoc::broken_intra_doc_links`
  |
  = note: `#[warn(renamed_and_removed_lints)]` on by default

...

warning: lint `broken_intra_doc_links` has been renamed to `rustdoc::broken_intra_doc_links`
 --> /Users/ero/workspace/rust-lightning/lightning-invoice/src/lib.rs:6:9
  |
6 | #![deny(broken_intra_doc_links)]
  |         ^^^^^^^^^^^^^^^^^^^^^^ help: use the new name: `rustdoc::broken_intra_doc_links`
  |
  = note: `#[warn(renamed_and_removed_lints)]` on by default

...

warning: lint `broken_intra_doc_links` has been renamed to `rustdoc::broken_intra_doc_links`
 --> /Users/ero/workspace/rust-lightning/lightning-persister/src/lib.rs:3:9
  |
3 | #![deny(broken_intra_doc_links)]
  |         ^^^^^^^^^^^^^^^^^^^^^^ help: use the new name: `rustdoc::broken_intra_doc_links`
  |
  = note: `#[warn(renamed_and_removed_lints)]` on by default

...

warning: lint `broken_intra_doc_links` has been renamed to `rustdoc::broken_intra_doc_links`
  --> /Users/ero/workspace/rust-lightning/lightning-net-tokio/src/lib.rs:69:9
   |
69 | #![deny(broken_intra_doc_links)]
   |         ^^^^^^^^^^^^^^^^^^^^^^ help: use the new name: `rustdoc::broken_intra_doc_links`
   |
   = note: `#[warn(renamed_and_removed_lints)]` on by default

...

warning: lint `broken_intra_doc_links` has been renamed to `rustdoc::broken_intra_doc_links`
  --> /Users/ero/workspace/rust-lightning/lightning-block-sync/src/lib.rs:16:9
   |
16 | #![deny(broken_intra_doc_links)]
   |         ^^^^^^^^^^^^^^^^^^^^^^ help: use the new name: `rustdoc::broken_intra_doc_links`
   |
   = note: `#[warn(renamed_and_removed_lints)]` on by default

[tnull]

Whoops, the changes don't seem to be compatible with the minimum supported version. So feel free to close this.

[TheBlueMatt]

No reason we can't take just the transmute fix.

[tnull]

True, 5490366 fixes only this warning.

[codecov-commenter]

Codecov Report

Merging #1249 (5490366) into main (34cdca9) will decrease coverage by 0.00%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main    #1249      +/-   ##
==========================================
- Coverage   90.41%   90.41%   -0.01%     
==========================================
  Files          70       70              
  Lines       38087    38087              
==========================================
- Hits        34437    34436       -1     
- Misses       3650     3651       +1     

Impacted Files	Coverage Δ
lightning-invoice/src/lib.rs	87.50% <0.00%> (ø)
lightning/src/ln/functional_tests.rs	97.28% <0.00%> (-0.02%)	⬇️

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