runit.h: change RUNIT define.

[ericonr]

Currently, the runit template in void-packages vseds this to /usr/bin/runit.

@sgn pointed out that leaving it as /sbin/runit is better because it allows for distros that haven't gone through usrmerge to still use this project. On the other hand, is there a chance that /sbin on someone's system doesn't point to /usr/bin, making this change still necessary, either here or in the template?

We could make it a proper build time configuration, as well.