-Werror=deprecated-copy (GCC trunk towards GCC 9)

(the SvxNumberType ctors/dtor update static nRefCount, so the copy ctor cannot
be defaulted)

Change-Id: Ibccc8377bf0f226d092cd53917b396fcd19cb4e0
Reviewed-on: https://gerrit.libreoffice.org/58167
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

include/editeng/numitem.hxx

@@ -65,6 +65,7 @@ public:
     explicit SvxNumberType(SvxNumType nType = SVX_NUM_ARABIC);
     SvxNumberType(const SvxNumberType& rType);
     ~SvxNumberType();
+    SvxNumberType & operator =(SvxNumberType const &) = default;
 
     OUString        GetNumStr( sal_Int32 nNo ) const;
     OUString        GetNumStr( sal_Int32 nNo, const css::lang::Locale& rLocale ) const;