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

Change-Id: I2fbe6376a1cf98d328464556917638a5679641d2
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/20354
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>

src/include/thread.h

@@ -21,7 +21,7 @@
 #include <timer.h>
 #include <arch/cpu.h>
 
-#if CONFIG_COOP_MULTITASKING && !defined(__SMM__) && !defined(__PRE_RAM__)
+#if IS_ENABLED(CONFIG_COOP_MULTITASKING) && !defined(__SMM__) && !defined(__PRE_RAM__)
 
 struct thread {
 	int id;