CMake: add -Wno-missing-braces to CXXFLAGS

This warning interferes with *correct* initializer list usage
with std::array.

See:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=25137
Merge pull request #42 from toxeus/noMissingBraces

CMake: add -Wno-missing-braces to CXXFLAGS