Put allow(dead_code) on oneof case enum try_from

This being unused in some cases is causing a warning at head, just suppressing the warning rather than conditionally emitting it only when we need it.

PiperOrigin-RevId: 668937899

src/google/protobuf/compiler/rust/oneof.cc

@@ -187,6 +187,7 @@ void GenerateOneofDefinition(Context& ctx, const OneofDescriptor& oneof) {
       impl $case_enum_name$ {
         //~ This try_from is not a TryFrom impl so that it isn't
         //~ committed to as part of our public api.
+        #[allow(dead_code)]
         pub(crate) fn try_from(v: u32) -> Option<$case_enum_name$> {
           match v {
             0 => Some($case_enum_name$::not_set),