Fix conditional pragma to work with 6.12

System/Posix.hs

System/Posix/Directory.hsc

System/Posix/DynamicLinker/Module.hsc

System/Posix/DynamicLinker/Prim.hsc

System/Posix/Env.hsc

System/Posix/Files.hsc

System/Posix/IO.hsc

System/Posix/Process.hsc

System/Posix/Resource.hsc

System/Posix/Semaphore.hsc

System/Posix/SharedMem.hsc

System/Posix/Signals.hsc

System/Posix/Signals/Exts.hsc

System/Posix/Temp.hsc

System/Posix/Terminal.hsc

System/Posix/Time.hsc

System/Posix/Unistd.hsc

System/Posix/User.hsc