Fix a compile problem when using -fno-common.

Thanks to olivier@freebsd.org for reporting the issue.

gtests/net/packetdrill/tcp_options.h

@@ -108,7 +108,7 @@ struct tcp_option {
 			u8 cookie[MAX_TCP_FAST_OPEN_COOKIE_BYTES];
 		} fast_open;
 	} data;
-} __packed tcp_option;
+} __packed;
 
 /* Allocate a new options list. */
 extern struct tcp_options *tcp_options_new(void);