Add a nested include of <sys/linker_set.h> to make the sysctl(9) manpage

accurate.  <sys/linker_set.h> is one of the very few headers similar to
<sys/queue.h> for which nested includes is allowed.

PR:		docs/153654
Reviewed by:	bde
MFC after:	1 month

sys/sys/sysctl.h

@@ -115,6 +115,8 @@ struct ctlname {
 #define CTL_AUTO_START	0x100
 
 #ifdef _KERNEL
+#include <sys/linker_set.h>
+
 #define SYSCTL_HANDLER_ARGS struct sysctl_oid *oidp, void *arg1, int arg2, \
 	struct sysctl_req *req