[Module::Build] add_build_element Should Add Only Once

[David E. W. <da...@ki...>]

Module::Builders,

Somehow, I've been finding in my tests that the same element keeps  
getting added to my build objets over and over again. I don't know  
why, but the fix is easy.

The attached patch modifies add_build_element() so that it only adds  
the element if it doesn't already exist. Any reason *not* to do this?

Thanks,

David