Don't install pst2dii manual when pst2dii is not enabled

Suggested-by: Milan Crha <mcrha@redhat.com>
Suggested-in: https://src.fedoraproject.org/rpms/libpst/c/aa52f8376a4a2a8dd8bd8ba301e294be0b679dd6?branch=rawhide
See-also: https://src.fedoraproject.org/rpms/libpst/pull-request/3
See-also: https://bugzilla.redhat.com/show_bug.cgi?id=1913613