They almost certainly ate herbs in salads and stews, and herbs like cilantro and thyme and parsley have a lot of vitamin C. So do many fruits like strawberries and blueberries. Perhaps most important, though, are brassicas. Broccoli, cabbage, kohlrabi, and turnips all are rich in C (as well as a lot of other nutrients.

So yes, the bulk of their food was meat, but they were also foraging for plants, fruits and nuts to supplement their diet - for the sake of variety if nothing else.

Interestingly, this discusses why eating meat and very few carbs prevents scurvy on its own. He cites some legit sources.