obs-ffmpeg: Fix warnings with FFmpeg 3.0.0+

Fixes warnings with deprecated function av_free_packet (which was
replaced by av_packet_unref)

plugins/obs-ffmpeg/obs-ffmpeg-compat.h

@@ -20,3 +20,6 @@
 # define av_frame_free avcodec_free_frame
 #endif
 
+#if LIBAVCODEC_VERSION_MAJOR >= 57
+#define av_free_packet av_packet_unref
+#endif