LinuxKPI: add the "dummy" includes directory to builds

[bz]

Summary

While we could add the summy includes directory manually to only the
drivers needing it, it seems a lot easier to simply add it to all
without any expected harm.

This is needed for more drivers (and to remove some #ifdef in current
ones) with empty header files being present not yielding errors.

MFC after: 1 week

[• hselasky]

sys/conf/kern.pre.mk

296–297

use "\" here to break long line?

[bz]

Break the long line though in this file long lines are common.

[imp]

All LinuxKPI drivers should get this, I agree. And I think these changes do the right thing wrt that.

[imp]

Is here a comment somewhere about what the linuxpki/dummy/include directory does or what purpose it serves?
If not, now might be a good time to add one.

Also, kmod.mk (or is that kern.mk) is included for all kernel builds, I think, so you might not need this in two places, but that's a pre-existing problem. the whole kern.pre.mk, kern.post.mk and kern.mk mess needs to be reworked, which is definitely not your problem...