package/avahi: break circular dependencies

We currently have two circular dependency chains:

    avahi -> libglade -> libgtk2 -> cups -> avahi
    avahi -> libgtk3 -> cups -> avahi

The cups -> avahi dependency makes sense, as cups would be able to use
Bonjour and mDNS to find printers, so we want to keep that dependency.

The libgtk2 -> cups and libgtk3 -> cups dependencies also make sense, to
be able to offer cups in the print dialogs.

However, the avahi -> libglade and avahi -> libgtk3 dependencies do not
really make sense. As Thomas puts it:

    The avahi GUI programs seem really useless to me. On Debian/Ubuntu
    distributions, they are not even packaged within the main avahi
    packages, but as separate packages, probably indicating that they
    are not very commonly used.

So, we drop the avahi -> libglade and avahi -> libgtk3 dependencies, to
break the circular dependency chain.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
[Thomas: add comment in the .mk file.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>