Fix warning in Semaphore002

Author: Bodigrim

tests/Semaphore002.hs

@@ -6,7 +6,7 @@ import System.Posix
 main :: IO ()
 main = do
   sem <- semOpen "/test" OpenSemFlags {semCreate = True, semExclusive = False} stdFileMode 0
-  forkIO $ do
+  _ <- forkIO $ do
       threadDelay (1000*1000)
       semPost sem