decklink: Header cleanup

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

libavdevice/decklink_common_c.h

@@ -19,6 +19,8 @@
  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  */
 
+#include "libavutil/log.h"
+
 struct decklink_cctx {
     const AVClass *cclass;