Fix casts.

A gconstpointer is a "pointer to const void", so it should be cast to
"pointer to const XXX".  A "pointer to const pointer to plugin" is a
"plugin *const *".

C's declaration syntax is not one of its strong points.  There's a
reason why the cdecl program, and the Web site that uses it,
http://cdecl.org, exists (as the tag line says, "C gibberish <->
English"), and why I used it to make sure I got this working.

Change-Id: Ia29bb25d17a1255a06b4ace542643c4b494d4977
Reviewed-on: https://code.wireshark.org/review/25835
Reviewed-by: Guy Harris <guy@alum.mit.edu>

wsutil/plugins.c

@@ -93,7 +93,7 @@ free_plugin(gpointer data)
 static gint
 compare_plugins(gconstpointer a, gconstpointer b)
 {
-    return g_strcmp0((*(const plugin **)a)->name, (*(const plugin **)b)->name);
+    return g_strcmp0((*(plugin *const *)a)->name, (*(plugin *const *)b)->name);
 }
 
 static void