bad: Add missing plugin since

These predate current stable release. Was never spotted since they weren't built
on the CI

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/7277>

subprojects/gst-plugins-bad/ext/ldac/ldac-plugin.c

@@ -24,6 +24,12 @@
 #include "gstldacenc.h"
 #include <string.h>
 
+/**
+ * plugin-ldac:
+ *
+ * Since: 1.20
+ */
+
 static gboolean
 plugin_init (GstPlugin * plugin)
 {

subprojects/gst-plugins-bad/ext/svtav1/gstsvtav1enc.c

@@ -1121,6 +1121,12 @@ gst_svtav1enc_parse_parameters_string (GstSvtAv1Enc * svtav1enc)
   g_strfreev (key_values);
 }
 
+/**
+ * plugin-svtav1:
+ *
+ * Since: 1.24
+ */
+
 static gboolean
 plugin_init (GstPlugin * plugin)
 {

subprojects/gst-plugins-bad/ext/svthevcenc/gstsvthevcenc.c

@@ -2256,6 +2256,12 @@ gst_svthevc_enc_get_property (GObject * object, guint prop_id,
   GST_OBJECT_UNLOCK (encoder);
 }
 
+/**
+ * plugin-svthevcenc:
+ *
+ * Since: 1.18
+ */
+
 static gboolean
 plugin_init (GstPlugin * plugin)
 {