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>
diff --git a/include/linux/compat.h b/include/linux/compat.h
index e561ee3..7236b8d 100644
--- a/include/linux/compat.h
+++ b/include/linux/compat.h
@@ -25,6 +25,8 @@
 	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