Correct SPL use of ENV_WRITEABLE_LIST

This converts 1 usage of this option to the non-SPL form, since there is
no SPL_ENV_WRITEABLE_LIST defined in Kconfig

Signed-off-by: Simon Glass <sjg@chromium.org>

lib/hashtable.c

@@ -942,7 +942,7 @@ int himport_r(struct hsearch_data *htab,
 		e.data = value;
 
 		hsearch_r(e, ENV_ENTER, &rv, htab, flag);
-#if !CONFIG_IS_ENABLED(ENV_WRITEABLE_LIST)
+#if !IS_ENABLED(CONFIG_ENV_WRITEABLE_LIST)
 		if (rv == NULL) {
 			printf("himport_r: can't insert \"%s=%s\" into hash table\n",
 				name, value);