[libc++] Workaround unused variable warning in test

This only showed up in C++11/C++14 where the static_assert below was
ifdef'd out, and the variable was indeed unused.

libcxx/test/std/atomics/types.pass.cpp

@@ -34,7 +34,7 @@ template <class A>
 void
 test_atomic()
 {
-    A a;
+    A a; (void)a;
 #if TEST_STD_VER >= 17
     static_assert((std::is_same<typename A::value_type, decltype(a.load())>::value), "");
 #endif