common: s/stringstream/ostream/ in str_map

There is no need to specialize more than ostream : it only makes it
impossible to use cerr or cout as a parameter to str_map.

Reviewed-By: Christophe Courtaut <christophe.courtaut@gmail.com>
Signed-off-by: Loic Dachary <loic@dachary.org>
Merge pull request #1900 from dachary/wip-ostream

common: s/stringstream/ostream/ in str_map

Reviewed-By: Christophe Courtaut <christophe.courtaut@gmail.com>