Research shows that if it is a habit, it is definitely a sigh of boredom or there is hide glue that's attracting your dog. Boredom makes a dog develop such behaviors so once you notice the dog licking, try and get his attention. Try telling him "DON'T LICK!" or just a phrase that will catch his attention. I'd prefer a tin with coins so that when he starts to lick, you shake the tin vigorously.