Follow-up of 'Get WebRTC building for Mac Catalyst'

This CL adds a missing "else" branch to add :metal_objc if the
target is iOS/Catalist.

Originally suggested https://webrtc-review.googlesource.com/c/src/+/224200/5#message-b43a38aa326df0133ae8f8af71c443d0c05098a8.

Bug: webrtc:11516
Change-Id: I27746926a853cad9c2c7079f155dd858f04f1792
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/245400
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#35732}

sdk/BUILD.gn

@@ -1324,6 +1324,7 @@ if (is_ios || is_mac) {
           ":audio_objc",
           ":base_objc",
           ":default_codec_factory_objc",
+          ":metal_objc",
           ":native_api",
           ":native_video",
           ":peerconnectionfactory_base_objc",