gnrc_tcp: abs() -> labs() for int32_t
gnrc_tcp: abs() -> labs() for int32_t

Fixes compilation on msp430 and probably avr.

OTAkeys/RIOT,OTAkeys/RIOT,jasonatran/RIOT,OlegHahm/RIOT,kYc0o/RIOT,ant9000/RIOT,kYc0o/RIOT,ant9000/RIOT,kaspar030/RIOT,kaspar030/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,kYc0o/RIOT,jasonatran/RIOT,ant9000/RIOT,kaspar030/RIOT,miri64/RIOT,miri64/RIOT,kaspar030/RIOT,ant9000/RIOT,jasonatran/RIOT,authmillenon/RIOT,miri64/RIOT,OlegHahm/RIOT,OTAkeys/RIOT,kYc0o/RIOT,OlegHahm/RIOT,OTAkeys/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,miri64/RIOT,authmillenon/RIOT,authmillenon/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,jasonatran/RIOT,miri64/RIOT,ant9000/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,kaspar030/RIOT