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