libxml2: add several security patches

Fixes:
CVE-2015-1819 - The xmlreader in libxml allows remote attackers to cause
a denial of service (memory consumption) via crafted XML data, related
to an XML Entity Expansion (XEE) attack.
CVE-2015-7941 - out-of-bounds memory access.
CVE-2015-7942 - heap-buffer-overflow in xmlParseConditionalSections.
CVE-2015-8035 - DoS via crafted xz file.

All patches upstream.

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