Add support for the (skip) annotation on parameters or return values

This was discussed in bug 649657.

https://bugzilla.gnome.org/show_bug.cgi?id=649657

Signed-off-by: David Zeuthen <davidz@redhat.com>

girnode.h

@@ -147,6 +147,7 @@ struct _GIrNodeParam
   gboolean optional;
   gboolean retval;
   gboolean allow_none;
+  gboolean skip;
   gboolean transfer;
   gboolean shallow_transfer;
   GIScopeType scope;