ADT: Add SmallVector<>::emplace_back(): fixup

Add missing `void` return type from `!LLVM_HAS_VARIADIC_TEMPLATES` case
in r223201.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223202 91177308-0d34-0410-b5e6-96231b3b80d8