gobject: Mark a helper variable as const

It’s only used for argv values, which are not modified here.

https://bugzilla.gnome.org/show_bug.cgi?id=614684

gobject/gobject-query.c

@@ -120,7 +120,7 @@ main (gint   argc,
   gboolean gen_froots = 0;
   gboolean gen_tree = 0;
   gint i;
-  gchar *iindent = "";
+  const gchar *iindent = "";
 
   f_out = stdout;