We have not found evidence of hearths, fire-bricks, hearthstones or the like on "Viking"-style ships that have been excavated, but that doesn't necessarily mean that there was never any provision for a fire (we have remarkably few longships to do archaeology on).

If they had had a fire, the ships (being made of wood) would have needed something to contain the fire on; ships of the period I'm more familiar with had brick hearths for cooking purposes. But we don't seem to see that in Norse or Norse-style ships, or in contemporary depictions of ships (such as the Bayeux tapestry) that we have. Best guess is that because most voyages in that era were from point to point near shore, it would have been fairly easy to pull ashore and light fires if cooking needed to be done; longships usually didn't venture out in the fall/winter or in inclement weather in any case.