powerpc: Fix build error in paravirt.h

./arch/powerpc/include/asm/paravirt.h:83:44: error: implicit declaration
of function 'smp_processor_id'; did you mean 'raw_smp_processor_id'?

smp_processor_id is defined in linux/smp.h but it is not included.

The build error happens only when the patch is applied to 5.3 kernel but
it only works by chance in mainline.

Fixes: ca3f969d ("powerpc/paravirt: Use is_kvm_guest() in vcpu_is_preempted()")
Signed-off-by: Michal Suchanek <msuchanek@suse.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210120132838.15589-1-msuchanek@suse.de