Remove unnecessary c_str() calls from the samples

Pass wxStrings directly to wxString::Format("%s") and similar
pseudo-vararg functions, there is no need for c_str() there since
wxWidgets 2.9.

Closes https://github.com/wxWidgets/wxWidgets/pull/1009

samples/mediaplayer/mediaplayer.cpp

@@ -1438,8 +1438,8 @@ void wxMediaPlayerTimer::Notify()
                     "State:%s Loops:%i D/T:[%i]/[%i] V:%i%%",
                     videoSize.x,
                     videoSize.y,
-                    sPosition.c_str(),
-                    sDuration.c_str(),
+                    sPosition,
+                    sDuration,
                     currentMediaCtrl->GetPlaybackRate(),
                     wxGetMediaStateText(currentpage->m_mediactrl->GetState()),
                     currentpage->m_nLoops,