Move FALLTHROUGH macro to a separate header, and give it an RTC_ prefix

Bug: chromium:805946
Change-Id: Ibb5dce9af27d0e48c9aee6b0a860b6f62b3c76a0
Reviewed-on: https://webrtc-review.googlesource.com/46145
Commit-Queue: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Tommi <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21889}
diff --git a/modules/audio_coding/BUILD.gn b/modules/audio_coding/BUILD.gn
index 4ed6377..c9c5ff1 100644
--- a/modules/audio_coding/BUILD.gn
+++ b/modules/audio_coding/BUILD.gn
@@ -1080,6 +1080,7 @@
     "../../rtc_base:gtest_prod",
     "../../rtc_base:rtc_base_approved",
     "../../rtc_base:sanitizer",
+    "../../rtc_base/system:fallthrough",
     "../../system_wrappers:field_trial_api",
     "../../system_wrappers:metrics_api",
   ]