src/cpu: add IS_ENABLED() around Kconfig symbol references

Some of these can be changed from #if to if(), but that will happen
in a follow-on commmit.

Change-Id: I4e5e585c3f98a129d89ef38b26d828d3bfeac7cf
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/20356
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>

src/cpu/x86/lapic/boot_cpu.c

@@ -14,7 +14,7 @@
 #include <smp/node.h>
 #include <cpu/x86/msr.h>
 
-#if CONFIG_SMP
+#if IS_ENABLED(CONFIG_SMP)
 int boot_cpu(void)
 {
 	int bsp;