avahi: fix avahi-ui build with libgtk3

Fixes:
http://autobuild.buildroot.net/results/daa/daad247db16818f25ab33402e26e27257defbe13/
http://autobuild.buildroot.net/results/720/720e2c8a5eab8b47d2510fe03b4a90ec8beafc17/
http://autobuild.buildroot.net/results/02b/02b4ab9ee07707ee4a4d4ea2b9c67bee91b1392d/
http://autobuild.buildroot.net/results/819/81914317ce82dc1321484d8c2b65647f92aa6929/

And many others.

avahi-ui uses deprecated gtk functionality, but explicitly requests to
disable the deprecated API support in gtk - Breaking the build when building
against libgtk3.

Patch is taken from Fedora:
https://bugzilla.redhat.com/show_bug.cgi?id=1001676



Signed-off-by: Peter Korsgaard <peter@korsgaard.com>