nb/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: Id5bc8b75b1fa372f31982b8636f1efa4975b61a5
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/20346
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>

src/northbridge/intel/haswell/gma.c

@@ -30,7 +30,7 @@
 #include "chip.h"
 #include "haswell.h"
 
-#if CONFIG_CHROMEOS
+#if IS_ENABLED(CONFIG_CHROMEOS)
 #include <vendorcode/google/chromeos/chromeos.h>
 #endif