Use bool in tui_before_prompt

This changes tui_before_prompt to take a bool rather than an int.

gdb/ChangeLog
2019-12-20  Tom Tromey  <tom@tromey.com>

	* tui/tui-hooks.c (tui_before_prompt): Change parameter to bool.
	(tui_before_prompt, tui_normal_stop): Update.

Change-Id: I9c7f2b764748fe19621851dc4fed4775a6db211a