Fix another cdash warning

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>

src/tests/kernel/threads.cpp

@@ -56,6 +56,8 @@ static void tst1() {
         ts[i].join();
         std::cout << "finished " << i << "\n";
     }
+    #else
+    mk(a);
     #endif
 }