Expose ILBC codec in webrtc/api/audio_codecs/

BUG=webrtc:7834, webrtc:7840

Review-Url: https://codereview.webrtc.org/2951873002
Cr-Commit-Position: refs/heads/master@{#18803}
diff --git a/webrtc/modules/audio_coding/BUILD.gn b/webrtc/modules/audio_coding/BUILD.gn
index afd2720..30460ba 100644
--- a/webrtc/modules/audio_coding/BUILD.gn
+++ b/webrtc/modules/audio_coding/BUILD.gn
@@ -322,6 +322,7 @@
     ":legacy_encoded_audio_frame",
     "../..:webrtc_common",
     "../../api/audio_codecs:audio_codecs_api",
+    "../../api/audio_codecs/ilbc:audio_encoder_ilbc_config",
     "../../base:rtc_base_approved",
     "../../common_audio",
   ]