PGFService.hs: fix type error caused by change to PGF.graphvizParseTree

Note that some of the graphviz functions have backwards incompatible changes
that might also affect other clients of the PGF run-time library.

Also added graphvizDefaults and export it together with GraphvizOptions from 
the PGF run-time library.

src/runtime/haskell/PGF.hs

@@ -122,6 +122,8 @@ module PGF(
            graphvizBracketedString,
            graphvizAlignment,
            gizaAlignment,
+           GraphvizOptions(..),
+           graphvizDefaults,
  
            -- * Probabilities
            Probabilities,