Bug 1363857 - Add missing includes to FFmpegLibWrapper. r=gerald

This was working because of the unified build, but passing
--enable-av1 changed the source partitioning in such a way
that the referenced defines and symbols were no longer available.

MozReview-Commit-ID: CRjyhLtOhoM

--HG--
extra : rebase_source : bce3810fcff39669c4578f1b545fb619be691e38

dom/media/platforms/ffmpeg/FFmpegLibWrapper.cpp

@@ -7,6 +7,7 @@
 #include "MediaPrefs.h"
 #include "mozilla/PodOperations.h"
 #include "mozilla/Types.h"
+#include "PlatformDecoderModule.h"
 #include "prlink.h"
 
 #define AV_LOG_DEBUG    48

dom/media/platforms/ffmpeg/FFmpegLibWrapper.h

@@ -5,6 +5,7 @@
 #ifndef __FFmpegLibWrapper_h__
 #define __FFmpegLibWrapper_h__
 
+#include "mozilla/Attributes.h"
 #include "mozilla/Types.h"
 
 struct AVCodec;