Update libFuzzer fuzz instructions

[Prabhat1308]

Without this flag , following the usual routine generates this error

Output

error: Fuzz targets need cfg=hashes_fuzz
  --> src/bin/msg_ping_target.rs:20:1
   |
20 | compile_error!("Fuzz targets need cfg=hashes_fuzz");
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: Fuzz targets need cfg=secp256k1_fuzz
  --> src/bin/msg_ping_target.rs:23:1
   |
23 | compile_error!("Fuzz targets need cfg=secp256k1_fuzz");
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: could not compile `lightning-fuzz` (bin "msg_ping_target") due to 2 previous errors
Error: failed to build fuzz script: ASAN_OPTIONS="detect_odr_violation=0" RUSTFLAGS="-Cpasses=sancov-module -Cllvm-args=-sanitizer-coverage-level=4 -Cllvm-args=-sanitizer-coverage-inline-8bit-counters -Cllvm-args=-sanitizer-coverage-pc-table -Cllvm-args=-sanitizer-coverage-trace-compares --cfg fuzzing -Clink-dead-code -Zsanitizer=address -Cdebug-assertions -C codegen-units=1" "cargo" "build" "--manifest-path" "/Users/prabhatverma/projects/rust-lightning/fuzz/Cargo.toml" "--target" "aarch64-apple-darwin" "--release" "--config" "profile.release.debug=true" "--features" "libfuzzer_fuzz" "--bin" "msg_ping_target"

After introducing this , normal fuzz operation is observed

INFO: Seed: 4260578952
INFO: Loaded 1 modules   (614 inline 8-bit counters): 614 [0x1027ee270, 0x1027ee4d6), 
INFO: Loaded 1 PC tables (614 PCs): 614 [0x1027ee4d8,0x1027f0b38), 
INFO:        0 files found in /Users/prabhatverma/projects/rust-lightning/fuzz/corpus/msg_ping_target
INFO: -max_len is not provided; libFuzzer will not generate inputs larger than 4096 bytes
INFO: A corpus is not provided, starting from an empty corpus
#2      INITED cov: 6 ft: 6 corp: 1/1b exec/s: 0 rss: 51Mb
#4194304        pulse  cov: 6 ft: 6 corp: 1/1b lim: 4096 exec/s: 1398101 rss: 461Mb
#8388608        pulse  cov: 6 ft: 6 corp: 1/1b lim: 4096 exec/s: 1398101 rss: 465Mb
#16777216       pulse  cov: 6 ft: 6 corp: 1/1b lim: 4096 exec/s: 1398101 rss: 468Mb
^C
==57544== libFuzzer: run interrupted; exiting

[ldk-reviews-bot]

I've assigned @valentinewallace as a reviewer!
I'll wait for their review and will help manage the review process.
Once they submit their review, I'll check if a second reviewer would be helpful.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.11%. Comparing base (8d8b4ea) to head (3341d7c).
Report is 456 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3705      +/-   ##
==========================================
+ Coverage   88.55%   89.11%   +0.56%     
==========================================
  Files         149      155       +6     
  Lines      114470   121678    +7208     
  Branches   114470   121678    +7208     
==========================================
+ Hits       101365   108430    +7065     
+ Misses      10606    10584      -22     
- Partials     2499     2664     +165     

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[joostjager]

I ran into this as well, but it seems I don't need to add --cfg=fuzzing. Does it work for you without that?

[TheBlueMatt]

cargo fuzz should add cfg=fuzzing automatically for us, but it also doesn't really hurt for us to add it manually. Its also required when running cargo test in the fuzz crate, so IMO its fine to have it be explicit.

[TheBlueMatt]

Gonna go ahead and land this since there's little reason to nitpick over a redundant cfg.