Fix build for FFmpeg < 3.3

The constant AV_CODEC_ID_AV1 was introduced in FFmpeg 3.3. Add an ifdef
to support older versions.

Fixes #3939 <https://github.com/Genymobile/scrcpy/issues/3939>

app/src/demuxer.c

@@ -33,7 +33,12 @@ sc_demuxer_to_avcodec_id(uint32_t codec_id) {
         case SC_CODEC_ID_H265:
             return AV_CODEC_ID_HEVC;
         case SC_CODEC_ID_AV1:
+#ifdef SCRCPY_LAVC_HAS_AV1
             return AV_CODEC_ID_AV1;
+#else
+            LOGE("AV1 not supported by this FFmpeg version");
+            return AV_CODEC_ID_NONE;
+#endif
         case SC_CODEC_ID_OPUS:
             return AV_CODEC_ID_OPUS;
         case SC_CODEC_ID_AAC: