Use correct DLGPROC callback signature in Win64 build

The callback used with CreateDialogIndirect() must return INT_PTR, which
is 64 bits under Win64, not LONG, which is still 32 bits.

This fixes a gcc8 -Wcast-function-type warning and might fix a real bug
in Win64 build too.

src/msw/nativdlg.cpp

@@ -36,7 +36,7 @@
 // global functions
 // ---------------------------------------------------------------------------
 
-extern LONG APIENTRY
+extern INT_PTR APIENTRY
 wxDlgProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam);
 
 // ===========================================================================

src/msw/toplevel.cpp

@@ -51,7 +51,7 @@
 
 // NB: wxDlgProc must be defined here and not in dialog.cpp because the latter
 //     is not included by wxUniv build which does need wxDlgProc
-LONG APIENTRY
+INT_PTR APIENTRY
 wxDlgProc(HWND hDlg, UINT message, WPARAM wParam, LPARAM lParam);
 
 // ----------------------------------------------------------------------------
@@ -1242,7 +1242,7 @@ void wxTopLevelWindowMSW::OnActivate(wxActivateEvent& event)
 }
 
 // the DialogProc for all wxWidgets dialogs
-LONG APIENTRY
+INT_PTR APIENTRY
 wxDlgProc(HWND WXUNUSED(hDlg),
           UINT message,
           WPARAM WXUNUSED(wParam),