qt: the GUI module requires the network module to work properly

Running a simple Qt GUI application on an embedded systems without
QtNetwork installed doesn't work, the dynamic loader complains that
QtNetwork is not present.

While I'm not a Qt build system expert, it looks like this dependency
really exists, since the src/gui/gui.pri file in Qt sources contains
the following line:

embedded: QT += network

Which would mean that the GUI module needs the network module to
operate properly. And a few files in src/gui/embedded/ also include
some QtNetwork stuff. This isn't really surprising since the Qt QWS
mechanism uses communication between processes through a Unix socket.

Therefore, we add a dependency on the network module in the GUI
module. We use a 'select' dependency here, since it's might not be
really obvious to the user that the GUI module requires the network
one.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>