MINOR: stream_interface: add an init callback to sock_ops

This will be needed for some socket layers such as SSL. It's not used
at the moment.
diff --git a/include/types/stream_interface.h b/include/types/stream_interface.h
index 2c57acd..fb89f50 100644
--- a/include/types/stream_interface.h
+++ b/include/types/stream_interface.h
@@ -106,6 +106,7 @@
 };
 
 struct sock_ops {
+	int (*init)(struct stream_interface *, void *); /* init function */
 	void (*update)(struct stream_interface *);  /* I/O update function */
 	void (*shutr)(struct stream_interface *);   /* shutr function */
 	void (*shutw)(struct stream_interface *);   /* shutw function */