sys/intr.h: formally depend on machine/intr.h

sys/intr.h originally started life as an extract of arm's intr.h, and
this include was dropped in its place. Changes in flight want to add
some MD definitions that we'll use in the more MI parts of INTRNG.

Let's formally reverse the dependency now since this is way more
common in general. All of the includes switched in this change that I
spot-checked were in-fact wanting declarations historically included in
sys/intr.h anyways.

Reviewed by: andrew, imp, jrtc27, mhorne, mmel, olce
Differential Revision: https://reviews.freebsd.org/D47002