Replace RTC_WARN_UNUSED_RESULT with ABSL_MUST_USE_RESULT in audio_coding

Bug: webrtc:12336
Change-Id: Icae229b957c2bfcc410788179a504c576cfde151
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/201736
Reviewed-by: Ivo Creusen <ivoc@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#32995}
diff --git a/modules/audio_coding/BUILD.gn b/modules/audio_coding/BUILD.gn
index e440b43..ffd6027 100644
--- a/modules/audio_coding/BUILD.gn
+++ b/modules/audio_coding/BUILD.gn
@@ -368,8 +368,8 @@
     "../../rtc_base:rtc_base_approved",
     "../../rtc_base:sanitizer",
     "../../rtc_base/system:arch",
-    "../../rtc_base/system:unused",
   ]
+  absl_deps = [ "//third_party/abseil-cpp/absl/base:core_headers" ]
 }
 
 rtc_source_set("isac_common") {