cpu/intel: 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: Ie685bbbb1cbf06d32631ea40ad120b6f45374b2e
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/20336
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

src/cpu/intel/hyperthreading/intel_sibling.c

@@ -20,7 +20,7 @@
 #include <smp/spinlock.h>
 #include <assert.h>
 
-#if CONFIG_PARALLEL_CPU_INIT
+#if IS_ENABLED(CONFIG_PARALLEL_CPU_INIT)
 #error Intel hyper-threading requires serialized CPU init
 #endif