Make send_get_hello public, so we can use it in send_receive

adsbus/send.h

@@ -1,14 +1,13 @@
 #pragma once
 
-#include <stdbool.h>
-
+struct buf;
 struct flow;
 struct packet;
-struct peer;
 
 void send_init(void);
 void send_cleanup(void);
 void *send_get_serializer(char *);
+void send_get_hello(struct buf **, void *);
 void send_write(struct packet *);
 void send_print_usage(void);
 extern struct flow *send_flow;