libxml2: security bump to version 2.9.2

Fixes:
CVE-2014-3660 - billion laugh variant
CVE-2014-0191 - Do not fetch external parameter entities

Also add hash file.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>