vtysh: fix build against readline 6.3

readline 6.3 removes some old deprecated funnily-named types.  This
updates vtysh to use the new types so it builds again.

Reported-by: Joel Teichroeb <klusark@archlinux.invalid>
References: https://bugs.archlinux.org/task/39495
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
diff --git a/vtysh/vtysh.c b/vtysh/vtysh.c
index 34c3bd6..89b9b25 100644
--- a/vtysh/vtysh.c
+++ b/vtysh/vtysh.c
@@ -2212,9 +2212,9 @@
 vtysh_readline_init (void)
 {
   /* readline related settings. */
-  rl_bind_key ('?', (Function *) vtysh_rl_describe);
+  rl_bind_key ('?', (rl_command_func_t *) vtysh_rl_describe);
   rl_completion_entry_function = vtysh_completion_entry_function;
-  rl_attempted_completion_function = (CPPFunction *)new_completion;
+  rl_attempted_completion_function = (rl_completion_func_t *)new_completion;
 }
 
 char *