buildtools: only use the ABI based version script for non private libraries

buildtools: only use the ABI based version script for non private libraries

Private libraries should not expose the symbols from the public namespace,
as then we may conflict with external libraries.

E.g. if we build tdb as private library, we want to be sure we're using the
private library and symbols, even if we also have the system libtdb.so loaded
via some external modules.

metze