[Concurrency][Tests] Require libdispatch in the sleep test.

We need to explicitly require `libdispatch` here, otherwise the
Windows build will fail.

rdar://141348916

Author: al45tair

test/Concurrency/Runtime/sleep_executor.swift

@@ -2,6 +2,7 @@
 
 // REQUIRES: concurrency
 // REQUIRES: executable_test
+// REQUIRES: libdispatch
 
 // rdar://106849189 move-only types should be supported in freestanding mode
 // UNSUPPORTED: freestanding