emacs: use (system-name) instead of system-name

Fix the deprecation warning:

In notmuch-maildir-fcc-make-uniq-maildir-id:
emacs/notmuch-maildir-fcc.el:279:53:Warning: ‘system-name’ is an obsolete
    variable (as of 25.1); use (system-name) instead

I've used (system-name) since at least 2011, so it must have been
around quite a while.