Work around gcc 4.0.3 bug.

This appears to fix the sim-eng build on the gDapper build machines.

Basic problem is that LayerBuffer::OverlaySource has a constructor that
calls SurfaceFlinger.signalEvent().  SurfaceFlinger lists LayerBuffer
as a friend, but that's not enough to convince gcc that the embedded
OverlaySource class is also a friend.  I don't see a way to make them
friendly, so I marked signalEvent() as public.