PGF2.Expr fixes

It needs to be listed in gf.cabal
unsafePerformIO is only exported from System.IO.Unsafe, starting with ghc-7.8

gf.cabal

@@ -133,7 +133,7 @@ Library
 
   if flag(c-runtime)
     exposed-modules: PGF2
-    other-modules:   PGF2.FFI
+    other-modules:   PGF2.FFI PGF2.Expr
                      GF.Interactive2 GF.Command.Commands2
     hs-source-dirs:  src/runtime/haskell-bind
     build-tools:     hsc2hs

src/runtime/haskell-bind/PGF2/Expr.hsc

@@ -3,6 +3,7 @@
 
 module PGF2.Expr where
 
+import System.IO.Unsafe(unsafePerformIO)
 import Foreign
 import Foreign.C
 import qualified Text.PrettyPrint as PP