common: Drop linux/stringify.h from common header

Move this uncommon header out of the common header.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/include/configs/el6x_common.h b/include/configs/el6x_common.h
index fe63bca..f7b96e4 100644
--- a/include/configs/el6x_common.h
+++ b/include/configs/el6x_common.h
@@ -8,6 +8,8 @@
 #ifndef __EL6Q_COMMON_CONFIG_H
 #define __EL6Q_COMMON_CONFIG_H
 
+#include <linux/stringify.h>
+
 #define CONFIG_BOARD_NAME		EL6Q
 
 #include "mx6_common.h"