[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>

arch/ia64/kernel/machine_kexec.c

@@ -157,7 +157,7 @@ void arch_crash_save_vmcoreinfo(void)
 #endif
 #ifdef CONFIG_PGTABLE_3
 	VMCOREINFO_CONFIG(PGTABLE_3);
-#elif  CONFIG_PGTABLE_4
+#elif defined(CONFIG_PGTABLE_4)
 	VMCOREINFO_CONFIG(PGTABLE_4);
 #endif
 }