decklink: Header cleanup

This commit cleans up the decklink files' header usage so that they pass
checkheaders.

libavdevice/decklink_common.h

@@ -19,9 +19,15 @@
  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  */
 
-#include <DeckLinkAPIVersion.h>
+#include <pthread.h>
+#include <semaphore.h>
+#include <DeckLinkAPI.h>
+
+extern "C" {
+#include "libavformat/avformat.h"
 
 #include "decklink_common_c.h"
+}
 
 class decklink_output_callback;
 class decklink_input_callback;