utmp.h: Don't fail when utmpx.h isn't present

Following a comment on the mailing-list, I made utmp.h return -1
when it's disabled, the problem with that is that it prevents the
container from starting completely, which isn't quite what I wanted.

This change makes the function succeed, the container will therefore
start but without utmp handler.

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>