Move AudioDecoderOpus next to AudioEncoderOpus

All AudioDecoder subclasses have historically lived in NetEq, but they
fit better with the codec they wrap.

BUG=webrtc:4557
R=henrik.lundin@webrtc.org

Review URL: https://codereview.webrtc.org/1342933005 .

Cr-Commit-Position: refs/heads/master@{#9944}
diff --git a/webrtc/modules/audio_coding/BUILD.gn b/webrtc/modules/audio_coding/BUILD.gn
index 235eb3c..fd96219 100644
--- a/webrtc/modules/audio_coding/BUILD.gn
+++ b/webrtc/modules/audio_coding/BUILD.gn
@@ -662,7 +662,9 @@
 
 source_set("webrtc_opus") {
   sources = [
+    "codecs/opus/audio_decoder_opus.cc",
     "codecs/opus/audio_encoder_opus.cc",
+    "codecs/opus/interface/audio_decoder_opus.h",
     "codecs/opus/interface/audio_encoder_opus.h",
     "codecs/opus/interface/opus_interface.h",
     "codecs/opus/opus_inst.h",