runq: More selective includes of <sys/runq.h> to reduce pollution

<sys/proc.h> doesn't need <sys/runq.h>. Remove this include and add it
back for kernel files that relied on the pollution.

Reviewed by: kib
MFC after: 1 month
Event: Kitchener-Waterloo Hackathon 202506
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D45387

(cherry picked from commit c21c24adde9822a7f364e131ad7b0c4008723502)