powerpc: Add missing prototype for time_init()

The function time_init did not have a prototype defined in the time.h
header. Fix the following warning (treated as error in W=1):

  arch/powerpc/kernel/time.c:1068:13: error: no previous prototype for ‘time_init’

Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>