timekeeping: __timekeeping_set_tai_offset can be static

Yet again, the kbuild test robot saves the day, noting
I left out defining __timekeeping_set_tai_offset as
static. It even sent me this patch.
Reported-by: NFengguang Wu <fengguang.wu@intel.com>
Signed-off-by: NJohn Stultz <john.stultz@linaro.org>

kernel/time/timekeeping.c

@@ -582,7 +582,7 @@ s32 timekeeping_get_tai_offset(void)
  * __timekeeping_set_tai_offset - Lock free worker function
  *
  */
-void __timekeeping_set_tai_offset(struct timekeeper *tk, s32 tai_offset)
+static void __timekeeping_set_tai_offset(struct timekeeper *tk, s32 tai_offset)
 {
 	tk->tai_offset = tai_offset;
 	tk->offs_tai = ktime_sub(tk->offs_real, ktime_set(tai_offset, 0));