Require cl when compiling jabber-rtt.el

This is needed for setf in Emacs 24.2 and earlier.
Conditionally require sha1 in jabber-logon.el

I really don't understand why, but sometimes executing (require 'sha1)
from a byte-compiled file fails in Emacs 24, even though sha1 should be
provided automatically.  Let's hope this fixes the problem...