[GLIB] Fix clang build (-Werror,-Wunused-lambda-capture)

https://bugs.webkit.org/show_bug.cgi?id=255674

Reviewed by Michael Catanzaro.

Depending on the GStreamer version this fails to compile, just ensure `this` is
always used.

* Source/WebCore/Modules/mediastream/gstreamer/GStreamerDataChannelHandler.cpp:
(WebCore::GStreamerDataChannelHandler::onMessageData):

Canonical link: https://commits.webkit.org/263166@main

Source/WebCore/Modules/mediastream/gstreamer/GStreamerDataChannelHandler.cpp

@@ -334,6 +334,7 @@ void GStreamerDataChannelHandler::onMessageData(GBytes* bytes)
         return;
 
     postTask([this, client = m_client, bytes = GRefPtr<GBytes>(bytes)] {
+        UNUSED_VARIABLE(this); // Conditionally used in DC_MEMDUMP.
         if (!*client)
             return;
         gsize size = 0;