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.

src/fluxbox.cc

@@ -1359,7 +1359,7 @@ bool Fluxbox::validateClient(const WinClient *client) const {
     WinClientMap::const_iterator it =
         find_if(m_window_search.begin(),
                 m_window_search.end(),
-                Compose(bind2nd(equal_to<WinClient *>(), client),
+                Compose(bind2nd(equal_to<const WinClient *>(), client),
                         Select2nd<WinClientMap::value_type>()));
     return it != m_window_search.end();
 }