Although exported rdtk isn't really mature and stable yet therefore it is
not exported/installed anymore. In case FreeRDP is built without
static libraries librdtk is installed as part or the
FreeRDP-ShadowTargets exports as this is the only place it is used right
now.
If a target is linked against libraries with cmake
(target_link_libraries) and the libraries are not marked as PRIVATE
they are "exported" and in case a other target is linked against this
target it is also linked against *all* (not private) libraries.
Without declaring private libraries PRIVATE a lot of over linking
(linking against unneeded libraries) was done.
