Fix async_task_priority

This test imports Darwin directly so it fails on Linux with
```
error: no such module 'Darwin'
```

Author: etcwilde

test/Concurrency/async_task_priority.swift

@@ -3,6 +3,7 @@
 // RUN: %target-codesign %t/async_task_priority
 // RUN: %target-run %t/async_task_priority
 
+// REQUIRES: VENDOR=apple
 // REQUIRES: executable_test
 // REQUIRES: concurrency
 // REQUIRES: libdispatch