GN conversion of audio_decoder_unittests

BUG=webrtc 6036

Review-Url: https://codereview.webrtc.org/2180853006
Cr-Commit-Position: refs/heads/master@{#13588}
diff --git a/webrtc/modules/audio_coding/audio_coding.gni b/webrtc/modules/audio_coding/audio_coding.gni
index d346d47..13577f8 100644
--- a/webrtc/modules/audio_coding/audio_coding.gni
+++ b/webrtc/modules/audio_coding/audio_coding.gni
@@ -28,3 +28,4 @@
 }
 
 audio_coding_defines = audio_codec_defines
+neteq_defines = audio_codec_defines