xmlcatalog: hold libxml2-native dependency

xmlcatalog: hold libxml2-native dependency

Put libxml2-native dependency in this class and remove
it from recipes inheriting this class.

In fact, if a recipe inherits this class and does not have
libxml2-native, the xmlcatalog_sstate_postinst would fail.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>