Bug 1672901 - Fix failure to build with 11.0 macOS SDK. r=jya

Differential Revision: https://phabricator.services.mozilla.com/D94561

dom/media/platforms/apple/AppleDecoderModule.cpp

@@ -153,10 +153,12 @@ bool AppleDecoderModule::CanCreateVP9Decoder() {
 /* static */
 bool AppleDecoderModule::RegisterSupplementalVP9Decoder() {
   static bool sRegisterIfAvailable = []() {
+    if (__builtin_available(macos 10.16, *)) {
       if (VTRegisterSupplementalVideoDecoderIfAvailable) {
         VTRegisterSupplementalVideoDecoderIfAvailable(kCMVideoCodecType_VP9);
         return true;
       }
+    }
     return false;
   }();
   return sRegisterIfAvailable;