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/stddef.h

@@ -37,7 +37,7 @@ typedef unsigned int wint_t;
 #endif
 
 /* Work around non-writable data segment in execute-in-place romstage on x86. */
-#if defined(__PRE_RAM__) && CONFIG_ARCH_X86
+#if defined(__PRE_RAM__) && IS_ENABLED(CONFIG_ARCH_X86)
 #define MAYBE_STATIC
 #else
 #define MAYBE_STATIC static