[Axccel] Remove -Wno-missing-braces in build

Summary:
I originally added the -Wno-missing-braces flag because I thought it was
erroneously flagging std::array initializations. Now I realize the extra
braces really are desired for these initializations, so I'm turning the
warning flag back on.

Reviewers: jlebar

Subscribers: mgorny, parallel_libs-commits

Differential Revision: https://reviews.llvm.org/D27941

llvm-svn: 290137