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}

webrtc/modules/audio_coding/codecs/opus/opus.gypi

@@ -43,7 +43,9 @@
         '<(webrtc_root)',
       ],
       'sources': [
+        'audio_decoder_opus.cc',
         'audio_encoder_opus.cc',
+        'interface/audio_decoder_opus.h',
         'interface/audio_encoder_opus.h',
         'interface/opus_interface.h',
         'opus_inst.h',