Since version 2.9.2, libxml2 provides its own cmake module. Install it
to avoid packages built with cmake and requiring libxml2 to fall back to
cmake's FindLibXml2 module, which seems to detect host libxml2 instead
of the one in staging_dir.

Fixes #4917.

Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>