The easiest way would be to use a low-dropout voltage regulator. You SHOULD regulate the voltage to a stable 5 volts anyway, because you can hurt some electronics by providing them lower voltage than the expected 5.
That being said, you can't just go get any old voltage regulator, because most have a dropout voltage of 2v - 2.5v above their target, meaning they would require 7v to 7.5v to give you a stable 5v.
The solution is a LDO, Low Dropout Voltage Regulator. The KA278 2A regulator is more than enough, and you don't connect anything to the adjustment pin to get a stable 5v. They dropout at 5.5v, meaning they don't put out any power when the voltage is lower than this. The Shimano DH-3N70 provides a nominal 6v, so you'd only be providing USB power at just under nomial voltage. Here's a link to 4 for about $5 on eBay.
This all being said, you may not end up providing that much charge from this setup, so be forewarned if your electronics don't seem to benefit from this setup. However, I'd strongly advise against hooking it directly either, some electronics are easily fried by throwing variable voltages through their USB (5v) charging port. If your light has an off switch, then by switching the light off you can make sure you get charging when you want it.
Also, be advised that most phones require between 500mA and 1A (iPhones won't charge with less than 1A), which at 5V = 5 Watts of power. Your dynamo only puts out 3 Watts of power, and at 6V = 0.5A. 0.5A * 5V = 2.5 Watts of effective 5V output, so don't be expecting to charge your iPhone even with the light off. Most Androids take between 500mA and 900mA, but will allow the charging to occur slower with less wattage, where iPhones will not charge at all.