Minor simplification in tui_default_win_viewport_height
tui_default_win_viewport_height doesn't need to look at tui_win_list;
it can simply check the type directly.
gdb/ChangeLog
2019-08-15 Tom Tromey <tom@tromey.com>
* tui/tui-layout.c (tui_default_win_viewport_height): Don't
examine tui_win_list.