avutil/buffer_internal: Include internal for buffer_size_t

Fixes checkheaders.

Reviewed-by: James Almer <jamrial@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavutil/buffer_internal.h

@@ -22,6 +22,7 @@
 #include <stdatomic.h>
 #include <stdint.h>
 
+#include "internal.h"
 #include "buffer.h"
 #include "thread.h"