Darcs fix compile

patches/darcs-2.16.5.patch

@@ -268,3 +268,15 @@ index 72ea398..afe755d 100644
                        http-types        >= 0.12.1 && < 0.12.4
  
      if flag(warn-as-error)
+diff --git a/src/Darcs/Util/Tree/Monad.hs b/src/Darcs/Util/Tree/Monad.hs
+index f639766..8947aea 100644
+--- a/src/Darcs/Util/Tree/Monad.hs
++++ b/src/Darcs/Util/Tree/Monad.hs
+@@ -35,6 +35,7 @@ module Darcs.Util.Tree.Monad
+ 
+ import Darcs.Prelude hiding ( readFile, writeFile )
+ 
++import Control.Monad
+ import Control.Exception ( throw )
+ 
+ import Darcs.Util.Path