Fix tests on stable for #[repr(packed(N))]

We need to use the nightly flag in generated bindings
because 1.33 isn't on stable and use of the attribute
will throw an error.

src/features.rs

@@ -199,8 +199,6 @@ rust_feature_def!(
     Nightly {
         /// `thiscall` calling convention ([Tracking issue](https://github.com/rust-lang/rust/issues/42202))
         => thiscall_abi;
-        /// repr(packed(N)) ([PR](https://github.com/rust-lang/rust/pull/57049))
-        => repr_packed_n;
     }
 );
 

tests/expectations/tests/issue-537-repr-packed-n.rs

@@ -6,6 +6,7 @@
     non_camel_case_types,
     non_upper_case_globals
 )]
+#![cfg(feature = "nightly")]
 
 /// This should not be opaque; we can see the attributes and can pack the
 /// struct.

tests/headers/issue-537-repr-packed-n.h

@@ -1,4 +1,4 @@
-// bindgen-flags: --rust-target nightly
+// bindgen-flags: --raw-line '#![cfg(feature = "nightly")]' --rust-target 1.33
 
 /// This should not be opaque; we can see the attributes and can pack the
 /// struct.