Use "class" for wxGTK wxWindowGesturesData definition

No real changes, just be consistent with wxWindowGesturesData forward
declaration as "class", clang (and MSVC) warn about mismatching keywords
being used for declaration and definition.

src/gtk/window.cpp

@@ -235,8 +235,9 @@ static bool gs_inSizeAllocate;
 #ifdef wxGTK_HAS_GESTURES_SUPPORT
 
 // Per-window data for gestures support.
-struct wxWindowGesturesData
+class wxWindowGesturesData
 {
+public:
     wxWindowGesturesData(wxWindow* win, GtkWidget *widget);
     ~wxWindowGesturesData();