testsuite: Fix warning introduced by nodiscard in libstdc++

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>

gcc/testsuite/ChangeLog:

	* g++.old-deja/g++.other/inline7.C: Cast nodiscard call to void.

gcc/testsuite/g++.old-deja/g++.other/inline7.C

@@ -8,7 +8,7 @@ std::list<int*> li;
 
 void f ()
 {
-  li.size ();
+  (void) li.size ();
 }
 
 int main ()