libs: window: don't add an unused function
libs: window: don't add an unused function

The macro GST_VAAPI_OBJECT_DEFINE_CLASS_WITH_CODE only defines
a function that is never used, thus when compiling we might see
this warning (clang):

gstvaapiwindow.c:147:1: warning: unused function 'gst_vaapi_window_class' [-Wunused-function]
GST_VAAPI_OBJECT_DEFINE_CLASS_WITH_CODE (GstVaapiWindow,
^

https://bugzilla.gnome.org/show_bug.cgi?id=759533

GStreamer/gstreamer-vaapi,ceyusa/gstreamer-vaapi,ceyusa/gstreamer-vaapi,GStreamer/gstreamer-vaapi