opencv: compile with -Wno-format-nonliteral
opencv plugin is pulling a header which makses clang++ 10 complain a lot and blocks -werror.
/usr/include/opencv4/opencv2/flann/logger.h:83:36: error: format string is not a string literal [-Werror,-Wformat-nonliteral]
int ret = vfprintf(stream, fmt, arglist);
^~~