[Freeciv-Dev] (PR#2928) queue_mapview_update called by gui-sdl

["Jason Short via RT" <rt@xxxxxxxxxxxxxx>]

At some point gui-sdl started calling queue_mapview_update().  This
means the changes I made to this function break compilation of the SDL
client.  But the SDL client really shouldn't be calling this function
anyway, since it never unqueues anything.  The unqueue is only done when
we're finished reading a series of network packets (since the queueing
is only don while we're reading those network packets).

Anyway, a fix is attached.  I'm not really sure why an
update_map_canvas_visible is needed at this point anyway, though...

jason