package/libiio: fix static build

Use the CMake module provided by libxml2 instead of the one from CMake to
correctly set the cflags and ldflags.

Fixes:
  http://autobuild.buildroot.net/results/964f98fafdcd7e994ea25cf0993915d81537c31c/

Note that this patch needs the following one to be merged in libxml2:
  https://mail.gnome.org/archives/xml/2014-December/msg00015.html



Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>