[cxx-interop] Add missing // REQUIRES: concurrency_runtime

This prevents this test from running on older macOS versions.

rdar://160960247

Author: egorzhdan

test/Interop/SwiftToCxx/class/swift-actor-execution.cpp

@@ -21,6 +21,7 @@
 // REQUIRES: executable_test
 
 // REQUIRES: concurrency
+// REQUIRES: concurrency_runtime
 
 #include <assert.h>
 #include "actor.h"