Fix warning in a future compiler version.

(Do not upgrade yet because of https://github.com/rust-lang/rust/issues/42101)

components/style/properties/longhand/font.mako.rs

@@ -1325,6 +1325,7 @@ ${helpers.single_keyword_system("font-kerning",
     }
 </%helpers:longhand>
 
+#[cfg(any(feature = "gecko", feature = "testing"))]
 macro_rules! exclusive_value {
     (($value:ident, $set:expr) => $ident:ident) => {
         if $value.intersects($set) {