fix warning in template Scalar::mul gcc < 4.8

In this particular case t shadows transpose of the base class Matx:
types.hpp:1805:14: warning: declaration of ‘t’ shadows a member of
'this' [-Wshadow]

Changelog gcc 4.8: The option -Wshadow no longer warns if a declaration
shadows a function declaration.

This warning is problematic because it prevents the module
opencv_contrib/modules/ruby to pass the build process

modules/core/doc/basic_structures.rst

@@ -534,7 +534,7 @@ Template class for a 4-element vector derived from Vec.
         template<typename T2> operator Scalar_<T2>() const;
 
         //! per-element product
-        Scalar_<_Tp> mul(const Scalar_<_Tp>& t, double scale=1 ) const;
+        Scalar_<_Tp> mul(const Scalar_<_Tp>& a, double scale=1 ) const;
 
         // returns (v0, -v1, -v2, -v3)
         Scalar_<_Tp> conj() const;