[Action][SKOS-Core] minor - added dct:issued props for all terms

I added a dct:issued statement for all terms, dated to the release 2004-03-26 
for all original terms.
-- 
Alistair Miles
Research Associate
CCLRC - Rutherford Appleton Laboratory
Building R1 Room 1.60
Fermi Avenue
Chilton
Didcot
Oxfordshire OX11 0QX
United Kingdom
Email: a.j.miles@rl.ac.uk
Tel: +44 (0)1235 445440