Explicitly default operator= for Vec<T, n>

Due to the explicitly declared copy constructor Vec<T, n>::Vec(Vec <T,n>&)
GCC 9 warns if there is no assignment operator, as having one typically
requires the other (rule-of-three, constructor/desctructor/assginment).

As the values are just a plain array the default assignment operator does
the right thing. Tell the compiler explicitly to default it.
Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>

modules/core/include/opencv2/core/matx.hpp

@@ -391,6 +391,10 @@ public:
     const _Tp& operator ()(int i) const;
     _Tp& operator ()(int i);
 
+#ifdef CV_CXX11
+    Vec<_Tp, cn>& operator=(const Vec<_Tp, cn>& rhs) = default;
+#endif
+
     Vec(const Matx<_Tp, cn, 1>& a, const Matx<_Tp, cn, 1>& b, Matx_AddOp);
     Vec(const Matx<_Tp, cn, 1>& a, const Matx<_Tp, cn, 1>& b, Matx_SubOp);
     template<typename _T2> Vec(const Matx<_Tp, cn, 1>& a, _T2 alpha, Matx_ScaleOp);