[Test][Concurrency] Add libdispatch to the requirements for the test.

The test uses Dispatch, mainly for convenience, but that means it
won't run everywhere.

rdar://156701386

Author: al45tair

test/Concurrency/Runtime/custom_task_executor_fast_path.swift

@@ -9,6 +9,7 @@
 // UNSUPPORTED: back_deployment_runtime
 // REQUIRES: concurrency_runtime
 // REQUIRES: synchronization
+// REQUIRES: libdispatch
 
 import StdlibUnittest
 import Synchronization