Fix mdbook-linkcheck license checks.

Author: ehuss

src/tools/tidy/src/deps.rs

@@ -55,6 +55,9 @@ const EXCEPTIONS: &[&str] = &[
     "sized-chunks",       // MPL-2.0+, cargo via im-rc
     // FIXME: this dependency violates the documentation comment above:
     "fortanix-sgx-abi",   // MPL-2.0+, libstd but only for `sgx` target
+    "dunce",              // CC0-1.0 mdbook-linkcheck
+    "codespan-reporting", // Apache-2.0 mdbook-linkcheck
+    "codespan",           // Apache-2.0 mdbook-linkcheck
 ];
 
 /// Which crates to check against the whitelist?