Fix List.foldl / Map.foldl ambiguosity

Fixes the following error:
src/runtime/haskell/PGF/Expr.hs:111:14:
    Ambiguous occurrence `foldl'
    It could refer to either `List.foldl',
                             imported from `Data.List' at src/runtime/haskell/PGF/Expr.hs:27:1-24
                             (and originally defined in `GHC.List')
                          or `Map.foldl',
                             imported from `Data.Map' at src/runtime/haskell/PGF/Expr.hs:28:1-40

src/runtime/haskell/PGF/Expr.hs

@@ -25,7 +25,7 @@ import PGF.Type
 import Data.Char
 import Data.Maybe
 import Data.List as List
-import Data.Map as Map hiding (showTree)
+import qualified Data.Map as Map hiding (showTree)
 import Control.Monad
 import qualified Text.PrettyPrint as PP
 import qualified Text.ParserCombinators.ReadP as RP