The Sri Lankan government Monday invited bids for its 15 per cent stake in Ruhunu Cement as part of its 2004 privatization program.  Bids should be submitted by Jan. 30, the Public Enterprises Reform Commission, or PERC, said in a statement posted on its Web site.  The Ruhunu Cement stake sale is part of government efforts to exit from minority shareholdings held by the treasury in several listed and unlisted firms.