Installing a strong-tie post anchor on unlevel concrete
One or two galvanized or stainless-steel washers between the post base and the concrete should do. The latter are thinner and would allow finer angle adjustment. If you don't like the look of the gap underneath, fill it with a fine bead of gray caulk. 

That said, there's probably plenty of slop against the post that you can plumb it and add the side screws to keep it plumb even if the base is slightly out of level.

To really make it solid with either approach, use heavy-duty construction adhesive between the base and the post.