Dependent packages fail if opm-core uses embedded TinyXML

If opm-core is built using the embedded TinyXML packages, other packages that uses Findopm-core.cmake will fail because they probe for TinyXML, but haven't been told to see the embedded directory inside the opm-core source.

Most likely fix is to take out the TinyXML-dependency from Findopm-core, but **not** from opm-core/CMakeLists.txt. Since we have an embedded version, we don't need to know whether TinyXML is found to put something in config.h; opm-core **always** has access to TinyXML.