swscale: fix compilation of bfin due to missing pixdesc.h header

This is required after sws_format_name() was replaced by
av_get_pix_fmt(), which is declared in libavutil/pixdesc.h.

libswscale/bfin/yuv2rgb_bfin.c

@@ -28,6 +28,7 @@
 #include <assert.h>
 #include "config.h"
 #include <unistd.h>
+#include "libavutil/pixdesc.h"
 #include "libswscale/rgb2rgb.h"
 #include "libswscale/swscale.h"
 #include "libswscale/swscale_internal.h"