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 pull request
https://github.com/Itseez/opencv_contrib/pull/116 to pass the build process