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

Commit d3f96f6610 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

sys/sys/sysctl.h

@@ -39,8 +39,8 @@
 #define	_SYS_SYSCTL_H_
 
 #ifdef _KERNEL
+#include <sys/queue.h>
 #include <sys/tree.h>
-#include <sys/systm.h>
 #endif
 
 /*