pulseaudio: correct libgtk3 handling

The gtk3 support uses X11 specific functionality, so it should only be
enabled if the X11 backend is enabled.

Fixes:
http://autobuild.buildroot.net/results/9d9/9d9ddea26a5193367a80eede1be1122cec5cd939/
http://autobuild.buildroot.net/results/3fb/3fbbdbf39e29319b349b2cf155397a121f24bf28/
http://autobuild.buildroot.net/results/ccf/ccf31740a0e730f2626db9051931b1e0703c770d/



And many others.

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