<sys/sysctl.h>: Reduce overly broad header pollution.

Commit d3f96f661050e9bd21fe29931992a8b9e67ff189 removed <sys/queue.h>
and replaced it with the very broad <sys/systm.h>. However, none of
the changes to sysctl.h in that commit require anything defined in
<sys/systm.h>. On the other hand, <sys/sysctl.h> does still make use
of queue macros. Drop the include of <sys/systm.h> and re-add
<sys/queue.h>.

Reviewed by: imp, kib, asomers
Obtained from: CheriBSD
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D37950