Add missing include
Add missing include

A complete type definition of `SimpleString` is needed for the
`TestPlugin::name_` member.

offa/cpputest,basvodde/cpputest,basvodde/cpputest,basvodde/cpputest,cpputest/cpputest,cpputest/cpputest,cpputest/cpputest,basvodde/cpputest,cpputest/cpputest,offa/cpputest,offa/cpputest,offa/cpputest