also allow unnecessary_clippy_cfg in Cargo.toml

This doesn't remove the `-A` parameter in the CI workflow, that's still
needed to override the blanket `-D clippy::all`. But it fixes the false
positive from normal `cargo clippy` invocations.

Author: hanna-kruppe

signal-hook-registry/Cargo.toml

@@ -23,3 +23,8 @@ libc = "^0.2"
 
 [dev-dependencies]
 signal-hook = { version = "~0.3", path = ".." }
+
+[lints.clippy]
+# actually necessary because Rust 1.26 doesn't support allow(clippy::foo) syntax
+unnecessary_clippy_cfg = "allow"
+