package/pulseaudio: fix build without avahi-client

Fix the following build failure without avahi-client raised since commit
7752afbe (before this commit, avahi was
silently disabled):

../output-1/build/pulseaudio-16.1/meson.build:754:2: ERROR: Dependency "avahi-client" not found, tried pkgconfig and cmake

Fixes:
 - http://autobuild.buildroot.org/results/0aea014eb4ed27870af908668d7b138d150f065a



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>