[NFC] Add { } to silence compiler warning [-Wmissing-braces].

../llvm/lib/Target/PowerPC/PPCISelLowering.cpp:5371:37: warning: suggest braces around initialization of subobject [-Wmissing-braces]
  std::array<EVT, 2> ReturnTypes = {MVT::Other, MVT::Glue};
                                    ^~~~~~~~~~~~~~~~~~~~~
                                    {                    }