[libc++] Add missing #include in <cwchar> tests

Thanks to Mikhail Maltsev for the patch.
Differential Revision: https://reviews.llvm.org/D63289

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@363290 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed