Input: add KEY_SPELLCHECK

HUT 1.12 defines Spell Check usage 0x1ab in Consumer page. There are
keyboards out there emitting this usage code (for example Microsoft
Natural Ergonomic Keyboard 4000). Add this key so that HID code could
map usages to it.

Acked-by: Dmitry Torokhov <dtor@mail.ru>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>