qxp::function_ref: reuse a constraint for the deduction guide

No functional change, just using the already-existing named constraint
instead of more enable_if fiddling.

Amends 29b65c98e7.

Pick-to: 6.7 6.6 6.5
Change-Id: Idae4962834f14e2a5273099b5611ebcbb2a54447
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

src/corelib/global/qxpfunctional.h

@@ -184,7 +184,7 @@ QT_SPECIALIZE_FUNCTION_REF(const, true );
 
 template <
     class F,
-    std::enable_if_t<std::is_function_v<F>, bool> = true
+    detail::if_function<F> = true
 >
 function_ref(F*) -> function_ref<F>;