Move private wxSpinCtrl helpers into a private header

Np real changes, just don't put these functions in the public
wx/spinctrl.h, they have nothing to do there.

src/generic/spinctlg.cpp

@@ -32,6 +32,8 @@
 
 #if wxUSE_SPINCTRL
 
+#include "wx/private/spinctrl.h"
+
 wxIMPLEMENT_DYNAMIC_CLASS(wxSpinDoubleEvent, wxNotifyEvent);
 
 // There are port-specific versions for the wxSpinCtrl, so exclude the