lavu/error: fix make checkheaders.

The errors need to be defined before including functions depending on
them. See av_size_mult() for instance. stddef.h is included for the
prototype of av_sterror (use of size_t).

libavutil/error.h

@@ -25,7 +25,7 @@
 #define AVUTIL_ERROR_H
 
 #include <errno.h>
-#include "avutil.h"
+#include <stddef.h>
 
 /**
  * @addtogroup lavu_error