vdpau: fix compilation on Libav

Libav's <libavcodec/vdpau.h> header uses some libavocdec symbols without
forward-declaring them and without including the headers declaring them.
FFmpeg's header for this is fine.

video/decode/vdpau.c

@@ -18,6 +18,7 @@
 #include <stddef.h>
 #include <assert.h>
 
+#include <libavcodec/avcodec.h>
 #include <libavcodec/vdpau.h>
 #include <libavutil/common.h>