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.

include/wx/private/spinctrl.h

@@ -0,0 +1,27 @@
+///////////////////////////////////////////////////////////////////////////////
+// Name:        wx/private/spinctrl.h
+// Purpose:     Private functions used in wxSpinCtrl implementation.
+// Author:      Vadim Zeitlin
+// Created:     2019-11-13
+// Copyright:   (c) 2019 Vadim Zeitlin <vadim@wxwidgets.org>
+// Licence:     wxWindows licence
+///////////////////////////////////////////////////////////////////////////////
+
+#ifndef _WX_PRIVATE_SPINCTRL_H_
+#define _WX_PRIVATE_SPINCTRL_H_
+
+namespace wxPrivate
+{
+
+// This is an internal helper function currently used by all ports: return the
+// string containing hexadecimal representation of the given number.
+extern wxString wxSpinCtrlFormatAsHex(long val, long maxVal);
+
+// The helper function to determine the best size for the given control.
+// We can't implement this function in the wxSpinCtrlBase because MSW implementation
+// of wxSpinCtrl is derived from wxSpinButton but uses the same algorithm.
+extern wxSize wxSpinCtrlGetBestSize(const wxControl* spin, int minVal, int maxVal, int base);
+
+} // namespace wxPrivate
+
+#endif // _WX_PRIVATE_SPINCTRL_H_