lavc/snow_dwt: add struct MpegEncContext to fix headers check.

add missing struct MpegEncContext, use make checkheaders
found this warning.
Signed-off-by: Jun Zhao <jun.zhao@intel.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/snow_dwt.h

@@ -24,6 +24,8 @@
 #include <stddef.h>
 #include <stdint.h>
 
+struct MpegEncContext;
+
 typedef int DWTELEM;
 typedef short IDWTELEM;