net/gcoap: cast required for pic32 platform
net/gcoap: cast required for pic32 platform

Fixes bug found during rework for sock_async implementation.

miri64/RIOT,OlegHahm/RIOT,basilfx/RIOT,kaspar030/RIOT,jasonatran/RIOT,OTAkeys/RIOT,ant9000/RIOT,kYc0o/RIOT,smlng/RIOT,miri64/RIOT,ant9000/RIOT,ant9000/RIOT,authmillenon/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,smlng/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,kaspar030/RIOT,yogo1212/RIOT,yogo1212/RIOT,OlegHahm/RIOT,authmillenon/RIOT,kaspar030/RIOT,jasonatran/RIOT,smlng/RIOT,yogo1212/RIOT,OlegHahm/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,yogo1212/RIOT,miri64/RIOT,ant9000/RIOT,kYc0o/RIOT,OTAkeys/RIOT,smlng/RIOT,yogo1212/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,smlng/RIOT,kYc0o/RIOT,OlegHahm/RIOT,basilfx/RIOT,basilfx/RIOT,ant9000/RIOT,authmillenon/RIOT,jasonatran/RIOT,basilfx/RIOT,miri64/RIOT,OTAkeys/RIOT,yogo1212/RIOT,authmillenon/RIOT,kaspar030/RIOT,miri64/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,jasonatran/RIOT,basilfx/RIOT