util/ScopeExit: use std::declval()

Fixes GCC 10 error:

 error: cannot call member function `Foo` without object

src/util/ScopeExit.hxx

@@ -50,7 +50,7 @@ public:
 	   "noexcept" declaration allows the destructor to throw if
 	   the function can throw; without this, a throwing function
 	   would std::terminate() */
-	~ScopeExitGuard() noexcept(noexcept(F::operator()())) {
+	~ScopeExitGuard() noexcept(noexcept(std::declval<F>()())) {
 		if (enabled)
 			F::operator()();
 	}