Fix warning from machine_kexec.c

commit c19ce0ab53ad9698968a154647f3dc22aad6c45b upstream.

Use proper cpp defined(...) constructs to avoid this:

arch/ia64/kernel/machine_kexec.c: In function 'arch_crash_save_vmcoreinfo':
arch/ia64/kernel/machine_kexec.c:160:8: warning: "CONFIG_PGTABLE_4" is not defined

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>