Input: add KEY_LOGOFF

HUT 1.12 defines Logoff usage 0x19c in Consumer page. There are
keyboards out there emitting this usage code (for example Microsoft
Wireless Laser Keyboard 6000). Add this key so that HID code could
map usages to it.

Signed-off-by: Khelben Blackstaff <eye.of.the.8eholder@gmail.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>