xml: Silence warnings with unknown XML tags
xml: Silence warnings with unknown XML tags

Silence the warnings that used to appear when the XML backend
encountered XML tags it didn't know about. As long as the DTD verifies
the XML, we can assume that the XML is sane and unknown XML tags are
just tags that were added by newer versions of IIOD.

Signed-off-by: Paul Cercueil <a027184a55211cd23e3f3094f1fdc728df5e0500@crapouillou.net>

analogdevicesinc/libiio,analogdevicesinc/libiio,analogdevicesinc/libiio,analogdevicesinc/libiio