Restore Thread.hpp in MediaPlayCtrl.cpp for the Windows Non-Release Ping Test (#16155)

The only use of Slic3r::create_thread in this file sits behind
!BBL_RELEASE_TO_PUBLIC && __WINDOWS__, so the include looked unused in
every configuration CI builds. Windows Debug and RelWithDebInfo builds
failed without it.

src/slic3r/GUI/MediaPlayCtrl.cpp

@@ -34,6 +34,7 @@
 #include <boost/nowide/utf8_codecvt.hpp>
 #include <map>
 #include "libslic3r/Utils.hpp"
+#include "libslic3r/Thread.hpp"
 #include <cstring>
 #include <boost/thread/lock_types.hpp>
 #include <memory>