net: Include byteorder in net6.h

This file uses byteorder but does not include the header file. Update
it.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/include/net6.h b/include/net6.h
index 2ceeaba..39573e4 100644
--- a/include/net6.h
+++ b/include/net6.h
@@ -11,6 +11,7 @@
 #define __NET6_H__
 
 #include <net.h>
+#include <asm/byteorder.h>
 #include <linux/ctype.h>
 #include <linux/errno.h>