cpu/rpx0xx: fix C++ compatibility for gpio
cpu/rpx0xx: fix C++ compatibility for gpio

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