MINOR: server: export new_server() function

The new_server() function will be useful to create a proxy for the
master-worker.
diff --git a/include/proto/server.h b/include/proto/server.h
index 1c2d1fd..75cba47 100644
--- a/include/proto/server.h
+++ b/include/proto/server.h
@@ -50,6 +50,7 @@
 int srv_set_addr_via_libc(struct server *srv, int *err_code);
 int srv_init_addr(void);
 struct server *cli_find_server(struct appctx *appctx, char *arg);
+struct server *new_server(struct proxy *proxy);
 
 /* functions related to server name resolution */
 int snr_update_srv_status(struct server *s, int has_no_ip);