Fix build breakage. Apparently all ARM configs build kern_et.c, but only a

few of them also build kern_clocksource.c.  That strikes me as insane, but
maybe there's a good reason for it.  Until I figure that out, un-break
the build by not referencing functions in kern_clocksource if NO_EVENTTIMERS
is defined.

sys/kern/kern_et.c

@@ -34,6 +34,8 @@ __FBSDID("$FreeBSD$");
 #include <sys/queue.h>
 #include <sys/timeet.h>
 
+#include "opt_timer.h"
+
 SLIST_HEAD(et_eventtimers_list, eventtimer);
 static struct et_eventtimers_list eventtimers = SLIST_HEAD_INITIALIZER(et_eventtimers);
 
@@ -121,7 +123,9 @@ void
 et_change_frequency(struct eventtimer *et, uint64_t newfreq)
 {
 
+#ifndef NO_EVENTTIMERS
 	cpu_et_frequency(et, newfreq);
+#endif
 }
 
 /*