kern_poll: include sys/sched.h

For sched_relinquish(). This fixes the build for some kernel configs.

Reported by:	Jenkins
Fixes:		1029dab634 ("mi_switch(): clean up switch types and their usage")

sys/kern/kern_poll.c

@@ -40,6 +40,7 @@ __FBSDID("$FreeBSD$");
 #include <sys/epoch.h>
 #include <sys/eventhandler.h>
 #include <sys/resourcevar.h>
+#include <sys/sched.h>
 #include <sys/socket.h>			/* needed by net/if.h		*/
 #include <sys/sockio.h>
 #include <sys/sysctl.h>