Do not activate YUVA422P in FFmpeg until 51.74.100

From 51.49.100 to 51.74.100, it only defines PIX_FMT_YUVA422P and not
AV_PIX_FMT_YUVA422P. There are other ways to fix, but ugly. And people should
update their FFmpeg anyway :)

modules/codec/avcodec/chroma.c

@@ -94,7 +94,7 @@ static const struct
     {VLC_CODEC_I422_10B, PIX_FMT_YUV422P10BE, 0, 0, 0 },
 
     {VLC_CODEC_YUV420A, PIX_FMT_YUVA420P, 0, 0, 0 },
-#if LIBAVUTIL_VERSION_CHECK( 51, 45, 0, 49, 100 )
+#if LIBAVUTIL_VERSION_CHECK( 51, 45, 0, 74, 100 )
     {VLC_CODEC_YUV422A, AV_PIX_FMT_YUVA422P, 0, 0, 0 },
 #endif