The supply for charging must be able to provide a regulated 5V DC at the necessary current, usually at least 500 mA. Typical bike dynamos don't/won't do that on their own, though I have heard of attachments which will provide USB compliant power from a dynamo. Others may know of specific devices they can recommend.

Here's one, but it's a bit pricy: Supernova ThePlug III Dynamo USB Charger - AEBike.com