[xmlsec] xmlsec dependency on libxml2

[Aleksey Sanin]

Currently xmlsec requires libxml2 version 2.4.2 or greater. However,
the fix for the following bug
	http://bugzilla.gnome.org/show_bug.cgi?id=142358
requires the latest and greatest libxml2 2.6.12

I clear understand that it is not a good idea to depend on the latest
version of some library. But I consider this bug very important and I
do want to have it fixed (thus the option of using ifdef's by libxml2
version is out).

Please speak now if you have very good reasons not to make this change
or forever hold your peace :)

Aleksey