Use xmllint built from source

Instead of relying on whatever version of xmllint is on the host system,
build and use the version in external/libxml2.

Test: diff build_aosp-marlin.ninja, expected changes.
Test: m $(xmllint targets in build_aosp-marlin.ninja)
Test: introduce xml error, build fails
Change-Id: I39579f06db3777e3b5c8dda7c7541c25a35887b2