fix warnings

Author: Saizan

io-sim/src/Control/Monad/IOSim/Internal.hs

@@ -2,6 +2,7 @@
 {-# LANGUAGE CPP                       #-}
 {-# LANGUAGE DerivingVia               #-}
 {-# LANGUAGE ExistentialQuantification #-}
+{-# LANGUAGE FlexibleContexts          #-}
 {-# LANGUAGE FlexibleInstances         #-}
 {-# LANGUAGE GADTSyntax                #-}
 {-# LANGUAGE MultiParamTypeClasses     #-}