[tcp] Add missing packed attribute on struct tcp_header

Debugged-by: Mark Rutland <mark.rutland@arm.com>
Debugged-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/ipxe/tcp.h

 	uint16_t win;		/* Advertised window */
 	uint16_t csum;		/* Checksum */
 	uint16_t urg;		/* Urgent pointer */
-};
+} __attribute__ (( packed ));
 
 /** @defgroup tcpopts TCP options
  * @{