PGFService: fix a type ambiguity when compiling without C run-time support

The function listCaches returns an empty list instead of the contents
of the cached C run-time system PGFs when compiled without C-run-time support
This gave rise to an ambiguity in GF.Server.

src/server/PGFService.hs

@@ -86,6 +86,8 @@ newPGFCache _ = do pgfCache <- newCache' PGF.readPGF
                    lblCache <- newCache' (fmap PGF.getDepLabels . readFile)
                    return $ Caches pgfCache lblCache
 flushPGFCache c = flushCache (pgfCache c)
+
+listPGFCache :: Caches -> IO ([(FilePath,UTCTime)],[(FilePath,UTCTime)])
 listPGFCache c = (,) # listCache (pgfCache c) % return []
 #endif