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

Author: marco-c

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

@@ -9898,6 +9898,25 @@ helpers
 :
 longhand
 >
+#
+[
+cfg
+(
+any
+(
+feature
+=
+"
+gecko
+"
+feature
+=
+"
+testing
+"
+)
+)
+]
 macro_rules
 !
 exclusive_value