Fix expat_impl.h.

Missing argument to XML_SetUnknownEncodingHandler
(compare to lib/expat.h) causes compile error.
Comment out as the fix would be more complex than
appropriate for a 3party library copy.
For some reasons e.g. gcc 6.3 seems to show no issue.

3party/expat/expat_impl.h

@@ -303,12 +303,15 @@ public:
 
   // @cmember Enable/Disable unknown encoding handler
 
+// disabled because it causes compile errors with gcc 7
+#if 0
   void EnableUnknownEncodingHandler (bool fEnable = true)
   {
     assert (m_p != NULL);
     XML_SetUnknownEncodingHandler (m_p,
       fEnable ? UnknownEncodingHandler : NULL);
   }
+#endif
 
   // @cmember Enable/Disable start namespace handler