Horror trends come and go over the decades. Zombies, slashers, vampires; every kind of nightmare-inducing ghoul has its heyday. (Often a few heydays!) But if there's one thing that horror movies have pretty much always agreed on, it's that nothing good ever happens while you're camping. You can now add Head Count to that proud horror tradition, a new chiller about a group of teenagers who head out to Joshua Tree, where their campfire ghost stories accidentally summon a shape-shifting creature when one of them recites an unknown chant they found online (tsk tsk!). Head …