servo: Merge #16950 - Fix warning in a future compiler version (from …

…servo:warn); r=nox

(Do not upgrade yet because of rust-lang/rust#42101)

Source-Repo: https://github.com/servo/servo
Source-Revision: dbd4adf3b266fb8d02cb717bc255c04f0fe41c05

UltraBlame original commit: f7050ffdfddb21a4b839f81b05bb356481fc4f7c

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

@@ -1315,6 +1315,7 @@ ${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) {