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

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

llvm-svn: 363290
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 64c1d456c1b21c7b942fd05337a2dc9241b108ab
2 files changed