Fix wbinfo --group-info if the winbind separator set to non \

Fix wbinfo --group-info if the winbind separator set to non \

In getgrsid_lookupsid_recv() we use parse_domain_user which itself looks at
lp_winbind_separator(). Thus when building up that group name we should better
use it as well.
(This used to be commit 5df75578ef1da41164936cd11b14114889201d47)