MINOR: connection: add a new destroy_bind_conf() entry to xprt_ops

This one will be set by the transport layers which want to destroy
a bind_conf. It will typically be used by SSL to release certificates,
CAs and so on.
diff --git a/include/types/connection.h b/include/types/connection.h
index 2fbc30e..6da2138 100644
--- a/include/types/connection.h
+++ b/include/types/connection.h
@@ -214,6 +214,7 @@
 	void (*close)(struct connection *);         /* close the transport layer */
 	int  (*init)(struct connection *conn);      /* initialize the transport layer */
 	int  (*prepare_bind_conf)(struct bind_conf *conf); /* prepare a whole bind_conf */
+	void (*destroy_bind_conf)(struct bind_conf *conf); /* destroy a whole bind_conf */
 	char name[8];                               /* transport layer name, zero-terminated */
 };