Use -Wno-missing-braces

[ezyang]

ATen uses {true, true, true} for boolean array literals, but unfortunately, Clang has a bug https://bugs.llvm.org/show_bug.cgi?id=21689 which makes it spuriously claim that more braces are needed as warning. We should turn off this warning.

I tried editing some obvious places in ATen's build system to apply this flag but it didn't stick. Maybe someone who is more familiar can fix it?