LIBKML: Manually add <?xml> node since libkml does not produce it currently and this is usefull in some circumstances (#5407)

git-svn-id: https://svn.osgeo.org/gdal/trunk@27009 f0d54148-0727-0410-94bb-9a71ac55c965
Merge branch 'trunk' into trunk_travis_macosx