clockevents: Make clockevents_subsys static

The clockevents_subsys struct is used for sysfs support and
is not declared or used outside the file it is defined in.
Fix the following warning by making it static:

kernel/time/clockevents.c:648:17: warning: symbol 'clockevents_subsys' was not declared. Should it be static?
Signed-off-by: NBen Dooks <ben.dooks@codethink.co.uk>
Cc: linux-kernel@lists.codethink.co.uk
Link: http://lkml.kernel.org/r/1466178974-7105-1-git-send-email-ben.dooks@codethink.co.ukSigned-off-by: NThomas Gleixner <tglx@linutronix.de>

kernel/time/clockevents.c

@@ -645,7 +645,7 @@ void tick_cleanup_dead_cpu(int cpu)
 #endif
 
 #ifdef CONFIG_SYSFS
-struct bus_type clockevents_subsys = {
+static struct bus_type clockevents_subsys = {
 	.name		= "clockevents",
 	.dev_name       = "clockevent",
 };