package/avahi: disable introspection

Fixes:
http://autobuild.buildroot.net/results/b9bf7cea8be9231552a10e8ea828bf24394402ba/



Building with introspection (together with D-Bus) support currently fails.
Fixing it is not trivial, so explicitly disable introspection for now.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Tested-by: Adam Duskett <aduskett@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>