powerpc/time: inline arch_vtime_task_switch()

arch_vtime_task_switch() is a small function which is called
only from vtime_common_task_switch(), so it is worth inlining

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>