Mark Xerces Dependencies as PRIVATE in CMake

Fixes https://issues.apache.org/jira/browse/XERCESC-2236, where trying
to use the generated CMake config package doesn't work because the
dependencies are not loaded using find_package in the config package.
This change assumes they're not necessary for users of the library and
marks them as PRIVATE so they don't end up in the config package in the
first place.
2 files changed