time: Make do_timer() and xtime_lock local to kernel/time/

All callers of do_timer() are converted to xtime_update(). The only
users of xtime_lock are in kernel/time/. Make both local to
kernel/time/ and remove them from the global header files.

[ tglx: Reuse tick-internal.h instead of creating another local header
  	file. Massaged changelog ]

Signed-off-by: Torben Hohn <torbenh@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: johnstul@us.ibm.com
Cc: yong.zhang0@gmail.com
Cc: hch@infradead.org
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

kernel/time/ntp.c

@@ -16,6 +16,8 @@
 #include <linux/mm.h>
 #include <linux/module.h>
 
+#include "tick-internal.h"
+
 /*
  * NTP timekeeping variables:
  */