[libc++abi] is_strcmp parameter to is_equal is unused for WIN32

Summary: Mark it as unused to avoid -Wunused-parameter.

Reviewers: EricWF, srhines, mstorsjo

Subscribers: christof, ldionne, libcxx-commits, cfe-commits

Differential Revision: https://reviews.llvm.org/D52368

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@342764 91177308-0d34-0410-b5e6-96231b3b80d8