services: elogind: Extend PAM to use 'pam_elogind.so'.

Thanks to Andy Wingo <wingo@igalia.com> for explaining this at
<https://lists.gnu.org/archive/html/guix-devel/2015-08/msg00439.html>.

* gnu/services/desktop.scm (pam-extension-procedure): New procedure.
(elogind-service-type): Extend PAM-ROOT-SERVICE-TYPE.