[Concurrency] Fix back deploy concurrency threading.

If building the stdlib/runtime is disabled, building BackDeployConcurrency
still needs the Threading object library.

Author: al45tair

stdlib/public/BackDeployConcurrency/CMakeLists.txt

@@ -65,4 +65,4 @@ set(LLVM_OPTIONAL_SOURCES
   SuspendingClock.swift
   TaskSleepDuration.swift)
 
-add_subdirectory(../Concurrency stdlib/public/BackDeployConcurrency)
+add_subdirectory(../Concurrency ../Threading stdlib/public/BackDeployConcurrency)