Concurrency: repair the build after swiftlang#34902

libdispatch is not part of the system on Linux and Windows, and dispatch
has not been used for the standard library up until this point.  The
current usage is limited to the Apple platforms, so rather than adding
another build of dispatch, conditionally include the header instead.

Author: compnerd

stdlib/public/Concurrency/Task.cpp

@@ -20,8 +20,10 @@
 #include "swift/Runtime/HeapObject.h"
 #include "TaskPrivate.h"
 
+#if defined(__APPLE__)
 // TODO: We shouldn't need this
 #include <dispatch/dispatch.h>
+#endif
 
 using namespace swift;
 using FutureFragment = AsyncTask::FutureFragment;