system: Use "@" to refer to the 'shadow' package.

Reported by Mark H Weaver
at <https://lists.gnu.org/archive/html/guix-devel/2017-06/msg00349.html>.

* gnu/system.scm (%base-packages): Use '@' for shadow.  This fixes an
error on Guile 2.0 where 'shadow' alone resolves to the (gnu system
shadow) module.

gnu/system.scm

@@ -490,7 +490,7 @@ explicitly appear in OS."
          lsof                                 ;for Guix's 'list-runtime-roots'
          pciutils usbutils
          util-linux inetutils isc-dhcp
-         shadow                                   ;for 'passwd'
+         (@ (gnu packages admin) shadow)          ;for 'passwd'
 
          ;; wireless-tools is deprecated in favor of iw, but it's still what
          ;; many people are familiar with, so keep it around.