gdb/tui: Don't cast between window types.

gdb/tui: Don't cast between window types.

Instead of casting between structure types to get the 'tui_gen_win_info'
info from a 'tui_win_info' access the generic member variable.  This is
inline with what is done throughout the rest of the tui code.

gdb/ChangeLog:

	* tui/tui-win.c (tui_set_focus): Use structure member 'generic'
	instead of casting the structure type.