thread/sema: name the lock temporary to silence a warning

test/thread/semaphore.cpp

@@ -11,8 +11,10 @@
 void
 fight (cruft::thread::semaphore &sem, const int iterations)
 {
-    for (int i = 0; i < iterations; ++i)
-        std::lock_guard {sem};
+    for (int i = 0; i < iterations; ++i) {
+        std::lock_guard tmp {sem};
+        (void)tmp;
+    }
 }