Fix build failure on Clang
Fix build failure on Clang

`operator<<(std::stringstream&)` returns `std::ostream&` which does not
have a `str()` member. libc++ has an implementation but it is
non-standard.

See <http://cplusplus.github.io/LWG/lwg-active.html#1203>

rbock/sqlpp17,rbock/sqlpp17