Fix compiler warning on OS X compile.
@@ -1176,7 +1176,7 @@ static int mg_vsnprintf(struct mg_connection *conn, char *buf, size_t buflen,
#ifdef __clang__
#pragma clang diagnostic push
- #pragma clang diagnostic ignored "Wformat-nonliteral"
+ #pragma clang diagnostic ignored "-Wformat-nonliteral"
/* Using fmt as a non-literal is intended here, since it is mostly called indirectly by mg_snprintf */
#endif