linux/compat.h: add dev_warn()

In order to prevent build errors for copied code from linux introduce
dev_warn().

Suggested-by: Scott Wood <oss@buserror.net>
Signed-off-by: Andreas Bießmann <andreas@biessmann.org>
Acked-by: Simon Glass <sjg@chromium.org>

include/linux/compat.h

@@ -25,6 +25,8 @@ extern struct p_current *current;
 	printf(fmt, ##args)
 #define dev_err(dev, fmt, args...)		\
 	printf(fmt, ##args)
+#define dev_warn(dev, fmt, args...)		\
+	printf(fmt, ##args)
 #define printk	printf
 #define printk_once	printf