powerpc/pseries: Fix little endian build with CONFIG_KEXEC=n

On ppc64le, builds with CONFIG_KEXEC=n fail with:

arch/powerpc/platforms/pseries/setup.c: In function ‘pseries_big_endian_exceptions’:
arch/powerpc/platforms/pseries/setup.c:403:13: error: implicit declaration of function ‘kdump_in_progress’
  if (rc && !kdump_in_progress())

This is because pseries/setup.c includes <linux/kexec.h>, but
kdump_in_progress() is defined in <asm/kexec.h>. This is a problem
because the former only includes the latter if CONFIG_KEXEC_CORE=y.

Fix it by including <asm/kexec.h> directly, as is done in powernv/setup.c.

Fixes: d3cbff1b5a90 ("powerpc: Put exception configuration in a common place")
Signed-off-by: Thiago Jung Bauermann <bauerman@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>