add signed integers to unnecessary_lints to ensure feature parity with clippy

Author: bend-n

tests/pass/shims/x86/intrinsics-x86-sse2.rs

@@ -1,5 +1,6 @@
 // We're testing x86 target specific features
 //@only-target: x86_64 i686
+#![allow(unnecessary_transmutes)]
 
 #[cfg(target_arch = "x86")]
 use std::arch::x86::*;