Make some TUI globals "static"

Make some TUI globals "static"

This changes a few TUI globals to be "static".  Tested by rebuilding.

gdb/ChangeLog
2019-12-30  Tom Tromey  <tom@tromey.com>

	* tui/tui-win.c (tui_border_mode_translate)
	(tui_border_kind_translate_vline, tui_border_kind_translate_hline)
	(tui_border_kind_translate_ulcorner)
	(tui_border_kind_translate_urcorner)
	(tui_border_kind_translate_llcorner)
	(tui_border_kind_translate_lrcorner, tui_active_border_mode)
	(tui_border_mode, tui_border_kind): Now static.

Change-Id: Ibb49a0df195dfe780a5ba1f90e9125ab5f6b7ce1