Avoid bogus assert after calling wxDatePickerCtrl::SetRange() in wxMSW.

If the old value didn't lie inside the new range, it was changed by the native
control internally but the value stored by wxDatePickerCtrl itself remained
unchanged, resulting in asserts later when the mismatch between them was
detected.

Closes #13189.

include/wx/msw/datectrl.h

@@ -60,6 +60,9 @@ protected:
     virtual bool MSWAllowsNone() const { return HasFlag(wxDP_ALLOWNONE); }
     virtual bool MSWOnDateTimeChange(const tagNMDATETIMECHANGE& dtch);
 
+private:
+    wxDateTime MSWGetControlValue() const;
+
     wxDECLARE_DYNAMIC_CLASS_NO_COPY(wxDatePickerCtrl);
 };