[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

llvm-svn: 342764