services: prosody: Fix activation script.

* gnu/services/messaging.scm (prosody-activation): Import (guix build utils)
  for 'mkdir-p'.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

gnu/services/messaging.scm

@@ -621,6 +621,7 @@ See also @url{http://prosody.im/doc/modules/mod_muc}."
                   (serialize-prosody-configuration config)))))
          (config-file (plain-file "prosody.cfg.lua" config-str)))
     #~(begin
+        (use-modules (guix build utils))
         (define %user (getpw "prosody"))
 
         (mkdir-p #$config-dir)