echo: Mark callback argument as unused

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

plugins/echo.c

@@ -41,7 +41,7 @@ static int recv_peer(int sd, char *buf, ssize_t len, struct sockaddr *sa, sockle
 	return len;
 }
 
-static int cb(int type)
+static int cb(int UNUSED(type))
 {
 	int sd = STDIN_FILENO;
 	char buf[BUFSIZ];