Makefile: Add -Werror to try-cflag so it works on clang
Makefile: Add -Werror to try-cflag so it works on clang

Clangs prints warnings when it sees unknown command line options. Adding
-Werror to the command line to upgrade them to errors results in clang
exiting with exit code 1 which is what try-cflag expects.

Signed-off-by: Oliver O'Halloran <3dfaff8fa6ae977f042064a112a6aac576279b9b@gmail.com>

open-power/skiboot,legoater/skiboot,shenki/skiboot,open-power/skiboot,qemu/skiboot,qemu/skiboot,qemu/skiboot,open-power/skiboot,open-power/skiboot,legoater/skiboot,shenki/skiboot,legoater/skiboot,qemu/skiboot,legoater/skiboot,shenki/skiboot,shenki/skiboot,open-power/skiboot,qemu/skiboot,legoater/skiboot,shenki/skiboot