add missing "require 'diff'" to Redmine::Helpers::Diff (#15105)

Contributed by Felix Bünemann.

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@12206 e93f8b46-1217-0410-a6f0-8f06a7374b81

lib/redmine/helpers/diff.rb

@@ -15,6 +15,8 @@
 # along with this program; if not, write to the Free Software
 # Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
 
+require 'diff'
+
 module Redmine
   module Helpers
     class Diff