Add check-cfg lint to fuzz to quiet warnings

Previously, the fuzzer would complain about unknown `cfg` flags. Here,
we add them to the list of allowed flags to quiet the many warnings.

Author: tnull

fuzz/Cargo.toml

@@ -49,3 +49,13 @@ opt-level = 1
 name = "lightning_fuzz"
 path = "src/lib.rs"
 crate-type = ["rlib", "dylib", "staticlib"]
+
+[lints.rust.unexpected_cfgs]
+level = "forbid"
+# When adding a new cfg attribute, ensure that it is added to this list.
+check-cfg = [
+    "cfg(fuzzing)",
+    "cfg(secp256k1_fuzz)",
+    "cfg(hashes_fuzz)",
+    "cfg(taproot)",
+]