keyboard: advertise KT_DEAD2 extended diacriticals

In addition to KT_DEAD which has limited support for diacriticals,
there is KT_DEAD2 that can support 256 criticals, so let's advertise
it in <linux/keyboard.h>.

This lets userland know abut the drivers/char/keyboard.c function
k_dead2, which supports more than the few trivial ones that k_dead
supports.

Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>