ntp: Fix prototype in the !CONFIG_GENERIC_CMOS_UPDATE case

In the !CONFIG_GENERIC_CMOS_UPDATE case the update_persistent_clock64() function
gets defined as a stub in ntp.c - make the prototype in <linux/timekeeping.h>
conditional on CONFIG_GENERIC_CMOS_UPDATE as well.

Fixes: 76e87d96 ("ntp: Consolidate the RTC update implementation")
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Thomas Gleixner <tglx@linutronix.de>

include/linux/timekeeping.h

@@ -303,6 +303,8 @@ extern int persistent_clock_is_local;
 extern void read_persistent_clock64(struct timespec64 *ts);
 void read_persistent_wall_and_boot_offset(struct timespec64 *wall_clock,
 					  struct timespec64 *boot_offset);
+#ifdef CONFIG_GENERIC_CMOS_UPDATE
 extern int update_persistent_clock64(struct timespec64 now);
+#endif
 
 #endif