Silence deprecation warning for av_lockmgr_register

It should not be used anymore - see
https://webrtc-review.googlesource.com/39503
but it can be removed only after the DEPS roll.

Bug: webrtc:8745
Change-Id: I1ee29948e99be6cb4a700b67d37f72a0747a9fc5
Reviewed-on: https://webrtc-review.googlesource.com/39505
Reviewed-by: Patrik Höglund <phoglund@webrtc.org>
Commit-Queue: Oleh Prypin <oprypin@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21612}

modules/video_coding/BUILD.gn

@@ -255,6 +255,11 @@ rtc_static_library("webrtc_h264") {
       "//third_party/ffmpeg:ffmpeg",
       "//third_party/openh264:encoder",
     ]
+
+    # TODO(bugs.webrtc.org/8745): stop using av_lockmgr_register
+    if (!is_win) {
+      cflags = [ "-Wno-deprecated-declarations" ]
+    }
   }
 }