We missed to set chess960 flag into the std::stringstream used to
setup the PV line.
Bug introduced with commit f803f33e6383fea1074aca2cf625983e148a6668
of 30/12/2010 when we started to print PV line into a std::stringstream
instead of directly into cout, where the chess960 flag is correctly set.
No functional change.
Signed-off-by: Marco Costalba <mcostalba@gmail.com>