lib: use libdbus for client's dbus messaging

Switch the dbus implementation for the client from systemd to
libdbus. The main reason is that, in flatpaks systemd is not easily
available. No phenomenological change for users of the library,
hopefully.

lib/meson.build

@@ -12,7 +12,7 @@ gamemode = shared_library(
         'client_impl.c',
     ],
     dependencies: [
-        dep_systemd,
+        dep_dbus,
     ],
     install: true,
     soversion: lt_current,