fix: hangup in plugin rescan

This adds a missing plugins_send_getmanifest() call in the rescan function
that lead to a RPC hangup. Not sure though if this is the proper fix.

Changelog-None

lightningd/plugin_control.c

@@ -141,6 +141,7 @@ plugin_dynamic_rescan_plugins(struct command *cmd)
 	if (res)
 		return res;
 
+	plugins_send_getmanifest(cmd->ld->plugins);
 	return command_still_pending(cmd);
 }