Clang errors, nullptr (C++14) incompatible with C++98

Add -Wno-c++98-compat to Clang flags to fix.

/home/melvin/Documents/mattermost/qt/test/mmqt/exec.cpp:12:21: error: 'nullptr' is incompatible with C++98 [-Werror,-Wc++98-compat]
                int b = mmqt_exec(nullptr, &res);
                                  ^