This unfortunately wouldn't work as the iPhone, once connected, will draw down the voltage of the panel and halt any charging activity. This is because a solar panel is electrically a current source, not a voltage source, and so cannot maintain its output voltage under heavy load.
When the iPhone is connected to a power source, it will attempt to draw current at 5 V from the source. Most iPhone chargers can supply more than 1 A current, and generic standard chargers will supply at least 500 mA. The iPhone, once connected, will draw all 250 mA that the panel can supply (it is attempting to draw even more current). When this happens, the solar panel voltage will drop out and charging will stop. This is because a solar panel can only operate on its I-V curve. For more info on the physics of why this happens, read up on the theory of solar cells.
When the panel current is at its maximum (250 mA), the voltage is 0 V. This is also known as the short-circuit current (Isc) operating condition. You can try this for yourself by connecting the positive and negative leads of the solar panel together and measuring the voltage between them. What you'd see on your iPhone setup is the charging starting, then stopping, then starting, then stopping ... resulting in no charging. To make this work, you need a larger panel plus some other components. There are a lot of Instructables that show you how to do this.