Ensure we build if a downstream crate sets --cfg=fuzzing

Downstream crates building fur fuzzing will usually set
`--cfg=fuzzing` as a side-effect of the Rust fuzzing tooling. Thus,
we should ensure we build without failure in such cases.

Author: TheBlueMatt

lightning/Cargo.toml

@@ -61,5 +61,10 @@ features = ["bitcoinconsensus", "secp-recovery"]
 [target.'cfg(ldk_bench)'.dependencies]
 criterion = { version = "0.4", optional = true, default-features = false }
 
+# Downstream crates building for fuzzing will usually set --cfg=fuzzing,
+# so we ensure build doesn't fail by taking the regex dependency explicitly.
+[target.'cfg(fuzzing)'.dependencies]
+regex = { version = "1.5.6" }
+
 [target.'cfg(taproot)'.dependencies]
 musig2 = { git = "https://github.com/arik-so/rust-musig2", rev = "27797d7" }