Fix Beastslayer Damage Bonus

Original patch by Zataron from old mangos community forums

This commit [e45f908](http://github.com/mangos-zero/server-old/commit/e45f908) is one of those pending backports from old mangos-zero concatenated in: https://github.com/cmangos/issues/wiki/classic_backporting-todo-zero-commits

I tested it and this is my:
> Tested: without this commit damages on beasts with a weapon are the same with or without the beastslayer enchantment. This commit fixes it by adding the right amount of bonus damage.