[S390] time: move local_tick_enable()/disable() to timex.h

Move the two functions to timex.h where they make more sense than in
hardirq.h.
No functional change.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>