Actually it is a vent for the failsafe pane.

There are two window panes- the outer, primary pane, which carries pressure all the time, and the inner, 'failsafe' pane, which is unloaded normally. The little hole lets cabin air into the space between the two, allowing the outer pane to hold the cabin pressure.

If the outer pane fails- and this happens once in a while- the only air that escapes immediately is the air in the gap. The little hole is too small to depressurize the airplane of course, so the failsafe pane keeps the airplane pressurized for the rest of the flight.