Include wx/stopwatch.h explicitly from wxTextCtrl unit test

Don't rely on it being implicitly included from another header when we
use wxStopWatch in this file (with wxGTK only).

tests/controls/textctrltest.cpp

@@ -26,6 +26,10 @@
 #include "wx/scopeguard.h"
 #include "wx/uiaction.h"
 
+#ifdef __WXGTK__
+    #include "wx/stopwatch.h"
+#endif
+
 #include "textentrytest.h"
 #include "testableframe.h"
 #include "asserthelper.h"