powerpc/kdump: Fix powernv build break when KEXEC_CORE=n

If KEXEC_CORE is not enabled, powernv builds fail as follows.

  arch/powerpc/platforms/powernv/smp.c: In function 'pnv_smp_cpu_kill_self':
  arch/powerpc/platforms/powernv/smp.c:236:4: error:
  	implicit declaration of function 'crash_ipi_callback'

Add dummy function calls, similar to kdump_in_progress(), to solve the
problem.

Fixes: 4145f358

 ("powernv/kdump: Fix cases where the kdump kernel can get HMI's")
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Balbir Singh <bsingharora@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>