uterm: add uterm_keysyms.h

uterm: add uterm_keysyms.h

We need a copy of xkbcommon-keysyms.h as long as xkbcommon is not included
in all major distributions. We _need_ this build-time dependency,
otherwise, we cannot build the other keyboard backends.

However, requiring xkbcommon as build-time dependency is not a solution as
no major distribution includes it.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>