[IA64] Fix warning from machine_kexec.c

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>