gnupg: security bump to version 1.4.18

Fixes CVE-2014-4617 (The do_uncompress function in g10/compress.c
allows context-dependent attackers to cause a denial of service
(infinite loop) via malformed compressed packets, as demonstrated by an
a3 01 5b ff byte sequence.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>