build-sys: make man/systemd.directives.xml depend on man/custom-entities.ent

build-sys: make man/systemd.directives.xml depend on man/custom-entities.ent

Currently, the following command sequence fails:

  make distclean
  ./autogen.sh c
  make distcheck

That's because the command invoked to build man/systemd.directives.xml needs
man/custom-entities.ent to function, which itself isn't a dependency.

The $(filter-out $<,$^) logic used to filter out everything from the
prerequisites except for the first word, which doesn't work anymore
now. Use $(SOURCE_XML_FILES) instead.