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/tsc/delay_tsc.c

@@ -136,7 +136,7 @@ void udelay(unsigned int us)
 	}
 }
 
-#if CONFIG_TSC_MONOTONIC_TIMER
+#if IS_ENABLED(CONFIG_TSC_MONOTONIC_TIMER)
 #include <timer.h>
 
 static struct monotonic_counter {