Include stddef from snow_dwt.h.

Fixes make checkheaders.

libavcodec/snow_dwt.h

@@ -21,6 +21,7 @@
 #ifndef AVCODEC_SNOW_DWT_H
 #define AVCODEC_SNOW_DWT_H
 
+#include <stddef.h>
 #include <stdint.h>
 
 typedef int DWTELEM;