osxaudio: Document GstOsxAudioElement interface

This is listed as a public interface implemented by osxaudio, so we
need to mark it as a plugin API so that it's listed in the
documentation correctly.

This is an ancient symbol, so add it to the symbol index too.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1601>

subprojects/gst-docs/symbols/symbol_index.json

@@ -12399,6 +12399,7 @@
   "GstOssSrc!src",
   "GstOssSrc:device",
   "GstOssSrc:device-name",
+  "GstOsxAudioElement",
   "GstOsxAudioSink!sink",
   "GstOsxAudioSrc!src",
   "GstOSXVideoSink!sink",

subprojects/gst-plugins-good/docs/gst_plugins_cache.json

@@ -10854,7 +10854,15 @@
         },
         "filename": "gstosxaudio",
         "license": "LGPL",
-        "other-types": {},
+        "other-types": {
+            "GstOsxAudioElement": {
+                "hierarchy": [
+                    "GstOsxAudioElement",
+                    "GInterface"
+                ],
+                "kind": "interface"
+            }
+        },
         "package": "GStreamer Good Plug-ins",
         "source": "gst-plugins-good",
         "tracers": {},

subprojects/gst-plugins-good/sys/osxaudio/gstosxaudioelement.c

@@ -85,6 +85,7 @@ gst_osx_audio_element_interface_init (GstOsxAudioElementInterface * iface)
 
   if (!initialized) {
     initialized = TRUE;
+    gst_type_mark_as_plugin_api (GST_OSX_AUDIO_ELEMENT_TYPE, 0);
   }
 
   /* default virtual functions */