Fix single point axis helpers

Just a tiny alteration, if I may, that solved a problem I was having with VMFII; the single point axis definitions that use axis(vecline) in the FGD (as opposed to axis(axis) for double ended helpers) weren't being recognized and adjusted, so after an embarrassing amount of stumbling around I figured out how to fix it.

This seems to solve the problem (see the included test maps), and it was a chance to finally learn GitHub pull requests, so I hope you'll indulge my forking your project over three lines of code. My ego wishes it were more.