gnrc: Fix unused function error when using LLVM
gnrc: Fix unused function error when using LLVM

RIOT-OS/RIOT,kaspar030/RIOT,kaspar030/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT