Charging while riding is much more complicated than you would imagine. There are a few gadgets you can get which will take the output of a generator and charge a battery, but the cheap ones are crappy and maybe dangerous. The good ones are surprisingly expensive. B&M makes a headlight that also provides USB charging output, but I believe you can only run the light or the charger, not both at once. And it's about $180, so it's justified only if you're going to use it a heck of a lot.

The battery on the light you bought is fairly big, and it needs a lot of charging current. To provide that much current with solar, you'd probably need panels covering the entire bike and your entire body. So as I said before, plug it in every night and consider getting a spare.