timers/sched_clock: Include local timekeeping.h for missing declarations

Include the timekeeping.h header to get the declaration of the
sched_clock_{suspend,resume} functions. Fixes the following sparse
warnings:

kernel/time/sched_clock.c:275:5: warning: symbol 'sched_clock_suspend' was not declared. Should it be static?
kernel/time/sched_clock.c:286:6: warning: symbol 'sched_clock_resume' was not declared. Should it be static?
Signed-off-by: Ben Dooks (Codethink) <ben.dooks@codethink.co.uk>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20191022131226.11465-1-ben.dooks@codethink.co.uk

kernel/time/sched_clock.c

@@ -17,6 +17,8 @@
 #include <linux/seqlock.h>
 #include <linux/bitops.h>
 
+#include "timekeeping.h"
+
 /**
  * struct clock_read_data - data required to read from sched_clock()
  *