package/cramfs: fix missing type

loff_t is guarded by _GNU_SOURCE. although already properly defined in
cramfsck, it was not defined in mkcramfs.

Fixes;
    http://autobuild.buildroot.org/results/57d/57de9e838fa953e2fb5e358cd535603fb249b7d2/
    http://autobuild.buildroot.org/results/e02/e023014934bf291add9d9dfdfc005bfec34dc1ff/


    [...]

Note: since cramfs was last released in 2002-02-24, we don't stand a
chance to get this merged any time soon... :-(

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>