vpx_config.c: add vpx_codec include

silences a missing prototype warning

Change-Id: I31a11c8390ae58e9a99ecc7cdf2a36334fac3f65

configure

@@ -726,6 +726,7 @@ CONFIGURE_ARGS="$@"
 process "$@"
 print_webm_license ${BUILD_PFX}vpx_config.c "/*" " */"
 cat <<EOF >> ${BUILD_PFX}vpx_config.c
+#include "vpx/vpx_codec.h"
 static const char* const cfg = "$CONFIGURE_ARGS";
 const char *vpx_codec_build_config(void) {return cfg;}
 EOF