[IO][ROOT-8425] Fix streamerinfo build check for string
[IO][ROOT-8425] Fix streamerinfo build check for string

since it is a special class: we are not interested in the actual
layout, it is opaque for the ROOT IO being it part of the STL.

olifre/root,karies/root,olifre/root,olifre/root,root-mirror/root,karies/root,olifre/root,root-mirror/root,root-mirror/root,root-mirror/root,root-mirror/root,karies/root,olifre/root,karies/root,root-mirror/root,root-mirror/root,olifre/root,olifre/root,karies/root,olifre/root,root-mirror/root,karies/root,root-mirror/root,karies/root,karies/root,olifre/root,karies/root,root-mirror/root,root-mirror/root,karies/root,olifre/root,olifre/root,karies/root