daemon/meson.build: Install dbus policy in /usr/share, not /etc

Gioele Barabucci requested to merge gioele/pulseaudio:dbus-policy-in-usr into master

From https://bugs.debian.org/1006631:

dbus supports policy files in both /usr/share/dbus-1/system.d and /etc/dbus-1/systemd. [The] recently released dbus 1.14.0, officially deprecates installing packages' default policies into /etc/dbus-1/systemd, instead reserving it for the sysadmin. This is the same idea as the difference between /usr/lib/udev/rules.d and /etc/udev/rules.d.

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