gl: Add since tags for new glbasesrc base class

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/merge_requests/647>

gst-libs/gst/gl/gstglbasesrc.c

@@ -39,6 +39,8 @@
  * context. It also provided some wrappers around #GstBaseSrc's `start()` and
  * `stop()` virtual methods that ensure an OpenGL context is available and
  * current in the calling thread.
+ *
+ * Since: 1.18
  */
 
 #define GST_CAT_DEFAULT gst_gl_base_src_debug

gst-libs/gst/gl/gstglbasesrc.h

@@ -48,6 +48,8 @@ GType gst_gl_base_src_get_type(void);
  * @running_time: the total running time
  *
  * The parent instance type of a base GStreamer GL Video source.
+ *
+ * Since: 1.18
  */
 struct _GstGLBaseSrc {
   GstPushSrc parent;
@@ -77,6 +79,8 @@ struct _GstGLBaseSrc {
  * @fill_gl_memory: called in the GL thread to fill the current video texture.
  *
  * The base class for GStreamer GL Video sources.
+ *
+ * Since: 1.18
  */
 struct _GstGLBaseSrcClass {
   GstPushSrcClass parent_class;