Add linter for missing "pragma once"

[jwnimmer-tri]

Every once in a while, a Drake PR or Drake-based PR is failing CI with a weird error, that we eventually track down to be a missing #pragma once in new (or worse, old!) code. It would be nice to update our cpplint.py to require the pragma.

[jwnimmer-tri]

I have a patch to a TRI fork of cpplint.py, that we could publish to RobotLocomotion/styleguide, if the platform review team likes the idea of enforcing this. What say you @EricCousineau-TRI @ggould-tri @sammy-tri @SeanCurtis-TRI @sherm1 @soonho-tri @RussTedrake? (Vote with sentiments on this post.)