guix: licenses: Add Artistic 2.0 license.

* guix/licenses.scm (artistic2.0): New variable.

guix/licenses.scm

@@ -28,7 +28,7 @@
             cc0
             cddl1.0
             cecill-c
-            clarified-artistic
+            artistic2.0 clarified-artistic
             cpl1.0
             epl1.0
             expat
@@ -129,6 +129,11 @@ which may be a file:// URI pointing the package's tree."
            "http://www.cecill.info/licences/Licence_CeCILL-C_V1-en.html"
            "https://www.gnu.org/licenses/license-list.html#CeCILL"))
 
+(define artistic2.0
+  (license "Artistic License 2.0"
+           "http://www.perlfoundation.org/artistic_license_2_0"
+           "http://www.gnu.org/licenses/license-list.html#ArtisticLicense2"))
+
 (define clarified-artistic
   (license "Clarified Artistic"
            ;; http://directory.fsf.org/wiki/User:Jgay/license-categorization#Clarified_Artistic_License