CodecOwner: Don't look at definitions for classes we don't link with
CodecOwner: Don't look at definitions for classes we don't link with

It's good hygiene and just generally the right thing to do. And
apparently at least sometimes required by Microsoft's compiler.

Review URL: https://codereview.webrtc.org/1364233002

Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#10060}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: f4d38ea57aa739b525066b095468cb4af1d2799b

sippet/webrtc,sippet/webrtc,sippet/webrtc,sippet/webrtc,sippet/webrtc,sippet/webrtc