[libc++abi] Remove redundant #include of <string.h>

<string.h> is always included, so we don't need the additional #include
only in the _WIN32 case.

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@373365 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed