[libcxx] Include <cstring> in tests that use strcmp

Reviewed as https://reviews.llvm.org/D56503.
Thanks to Andrey Maksimov for the patch.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@351847 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed