Change "win_resized" to bool

Change "win_resized" to bool

This changes the "win_resized" global to be a bool and then updates
the uses.

gdb/ChangeLog
2019-09-20  Tom Tromey  <tom@tromey.com>

	* tui/tui.c (tui_enable): Update.
	* tui/tui-win.c (tui_sigwinch_handler, tui_async_resize_screen):
	Update.
	* tui/tui-data.h (tui_win_resized, tui_set_win_resized_to):
	Update.
	* tui/tui-data.c (win_resized): Now bool.
	(tui_win_resized): Return bool.
	(tui_set_win_resized_to): Accept a bool.