systemd: Depend on dbus.service

Solution suggested by Xi Ruoyao.

The dbus user service is required for various features - the summary says:
'dbus (Bluetooth, rt, portal, pw-reserve)'

On session logout the dbus service gets shut down while the Pipewire one
relies on a timeout. If a user logs in again before PW timed out, the
later stays alive but doesn't handle re-connecting to the dbus service
of the new session, breaking the camera portal and potentially other
features.

Thus hard-depend on the dbus service (if enabled at build time) and thus
shut down together with it.

(cherry picked from commit 2625983a23)

src/daemon/systemd/user/meson.build

@@ -11,6 +11,12 @@ systemd_config = configuration_data()
 systemd_config.set('PW_BINARY', pipewire_bindir / 'pipewire')
 systemd_config.set('PW_PULSE_BINARY', pipewire_bindir / 'pipewire-pulse')
 
+pw_service_reqs = ''
+if get_option('dbus').enabled()
+  pw_service_reqs += 'dbus.service '
+endif
+systemd_config.set('PW_SERVICE_REQS', pw_service_reqs)
+
 configure_file(input : 'pipewire.service.in',
                output : 'pipewire.service',
                configuration : systemd_config,

src/daemon/systemd/user/pipewire.service.in

@@ -13,7 +13,7 @@ Description=PipeWire Multimedia Service
 #
 # After=pipewire.socket is not needed, as it is already implicit in the
 # socket-service relationship, see systemd.socket(5).
-Requires=pipewire.socket
+Requires=pipewire.socket @PW_SERVICE_REQS@
 ConditionUser=!root
 
 [Service]