pkg/littlefs2: bump version to 2.5.1
pkg/littlefs2: bump version to 2.5.1

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