gdb: tui_win_name: Make parameter and result const.

gdb: tui_win_name: Make parameter and result const.

This commit makes the parameter and the result for 'tui_win_name'
constant.  There's one place in the code that is then updated as a
result of this change.

gdb/ChangeLog:

	* tui/tui-data.c (tui_partial_win_by_name): Window name is const.
	(tui_win_name): Make parameter and result const.
	* tui/tui-data.h (tui_win_name): Make parameter and result const.