Don't cast a tui_win_info directly to tui_gen_win_info

Don't cast a tui_win_info directly to tui_gen_win_info

I found a few spots that directly cast a tui_win_info to a
tui_gen_win_info.  However, I think it's a bit better here to take the
address of the "generic" member.  As far as I know, nothing relies on
being able to downcast here, so this gives us the freedom to rearrange
the structure.

gdb/ChangeLog
2019-06-16  Tom Tromey  <tom@tromey.com>

	* tui/tui-wingeneral.c (tui_unhighlight_win, tui_highlight_win)
	(make_all_visible): Use address of member.