FindEXPAT: add a EXPAT_USE_STATIC_LIBS hint

The effect of that hint is to restrict the candidate library names, but
more essentially to set the `XML_STATIC` compile definitions which is
required to get proper linking.

Vcpkg-Issue: https://github.com/microsoft/vcpkg/issues/1100
GDAL-Issue: https://github.com/OSGeo/gdal/issues/7955