vp9_dx_iface.c uses CONFIG_VP9_POSTPROC but config.h not included

Change-Id: Id316b3786214bf1028992968955da917e3f2d4a3

vp9/vp9_dx_iface.c

@@ -11,6 +11,7 @@
 #include <stdlib.h>
 #include <string.h>
 
+#include "./vpx_config.h"
 #include "./vpx_version.h"
 
 #include "vpx/internal/vpx_codec_internal.h"