cmake: libxml2 detection: try first the CMake module from libxml2
cmake: libxml2 detection: try first the CMake module from libxml2

Libxml2 >=2.9.2 provides its own CMake module, so check for it before
falling back on the CMake's module FindLibXml2.cmake.

Signed-off-by: Samuel Martin <fa84b044cd972a3040f6596903a3209d1974a1c1@gmail.com>

sensarliar/libiio,Sunderfield/libiio,gburca/libiio,analogdevicesinc/libiio,analogdevicesinc/libiio,sensarliar/libiio,analogdevicesinc/libiio,Sunderfield/libiio,gburca/libiio,gburca/libiio,analogdevicesinc/libiio,sensarliar/libiio,Sunderfield/libiio