input-settings: Don't eat up seat reference

MetaInputSettings unref:ed the seat on destruction, but it never ref:ed
it on construction, meaning it "stole" the reference from the rightful
owner. Make MetaInputSettings less of a thief.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1775>

src/backends/meta-input-settings.c

@@ -175,7 +175,7 @@ meta_input_settings_set_property (GObject      *object,
   switch (prop_id)
     {
     case PROP_SEAT:
-      priv->seat = g_value_get_object (value);
+      priv->seat = g_value_dup_object (value);
       break;
     default:
       G_OBJECT_WARN_INVALID_PROPERTY_ID (object, prop_id, pspec);