[gdb/tui] Add default arguments to tui_rl_switch_mode

Add default arguments for tui_rl_switch_mode, to allow:
...
-tui_rl_switch_mode (0 /* notused */, 0 /* notused */);
+tui_rl_switch_mode ();
...

Tested on x86_64-linux.

Reviewed-By: Keith Seitz <keiths@redhat.com>