powerpc: Add missing prototype for hdec_interrupt

In commit dabe859e ("powerpc: Give hypervisor decrementer interrupts
their own handler") an empty body function was added, but no prototype
was declared. Fix warning (treated as error in W=1):

  arch/powerpc/kernel/time.c:629:6: error: no previous prototype for ‘hdec_interrupt’

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