Do not call deprecated av_register_all()

av_register_all() is deprecated in FFmpeg since this commit:
<http://git.videolan.org/?p=ffmpeg.git;a=commitdiff;h=0694d8702421e7aff1340038559c438b61bb30dd>

It is now useless to call it:
<https://ffmpeg.org/pipermail/ffmpeg-devel/2018-February/225051.html>

Fixes <https://github.com/Genymobile/scrcpy/issues/203>.

app/src/main.c

@@ -274,7 +274,9 @@ int main(int argc, char *argv[]) {
         return 0;
     }
 
+#if LIBAVCODEC_VERSION_INT < AV_VERSION_INT(58, 9, 100)
     av_register_all();
+#endif
 
     if (avformat_network_init()) {
         return 1;