check_headers: UNICODE is not a private define

src/check_headers

@@ -8,6 +8,7 @@ cd "$(dirname "$0")" || exit $?
 regexp="$(cat ../config.h.in | \
 	sed -n -e 's/^#undef \([A-Z0-9_]*\)$/\1/p' | \
 	grep -v 'WORDS_BIGENDIAN' | \
+	grep -v 'UNICODE' | \
 	grep -v '__LIBVLC__' | \
 	xargs | \
     sed -e 's/ /\\\(\\s\\\|$\\\)\\\| /g')"