avutil/imgutils: Don't include avutil.h

It is a remnant of an FF_API_* inclusion (back from when they were in
avutil.h and not in version.h).

Reviewed-by: Martin Storsjö <martin@martin.st>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavutil/imgutils.h

@@ -27,8 +27,10 @@
  * @{
  */
 
-#include "avutil.h"
+#include <stddef.h>
+#include <stdint.h>
 #include "pixdesc.h"
+#include "pixfmt.h"
 #include "rational.h"
 
 /**