package/pulseaudio: be explicit about udev usage

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
[Thomas: udev was already handled as an optional dependency, Bernd's
patch only made it explicit by using
--enable-libudev/--disable-libudev.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>