lavdev: include libavformat/avformat.h in avdevice.h

The header is always required for files which directly deal with
devices, since libavdevice uses the AVFormat* structures defined in
avformat.h.

Avoid the need to explicitely add libavformat/avformat.h.

libavdevice/avdevice.h

@@ -20,6 +20,7 @@
 #define AVDEVICE_AVDEVICE_H
 
 #include "libavutil/avutil.h"
+#include "libavformat/avformat.h"
 
 #define LIBAVDEVICE_VERSION_MAJOR 53
 #define LIBAVDEVICE_VERSION_MINOR  0