webrtc: Fix memory leak

Fix a leaked string when building RTX info.
diff --git a/ext/webrtc/gstwebrtcbin.c b/ext/webrtc/gstwebrtcbin.c
index 87ad7bb..cd31456 100644
--- a/ext/webrtc/gstwebrtcbin.c
+++ b/ext/webrtc/gstwebrtcbin.c
@@ -1505,6 +1505,7 @@
     str = g_strdup_printf ("%u", target_ssrc);
     gst_structure_set (trans->local_rtx_ssrc_map, str, G_TYPE_UINT,
         g_random_int (), NULL);
+    g_free (str);
 
     str = g_strdup_printf ("%u", pt);
     gst_sdp_media_add_format (media, str);