Constify the type passed to std::equal_to.

std::bind2nd() has problems with the existing call because it uses the
argument type passed to std::equal_to() in std::binder2nd()'s constructor,
but WinClient* does not have the const specifier that `client' does.

The call works fine with libstdc++ because of the way it implements
std::bind2nd(), but fails with libc++ and possibly with other STL
implementations as well.