xrootd: add XrdClient/XrdClientSid.hh private header to incl. path

Another xrootd private header XrdClient/XrdClientSid.hh is being used
by CMSSW and no more distributed. Adding it to ./include/xrootd/private.

Signed-off-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>
Merge pull request #150 from davidlt/xrootd-add-xrddlientsid-hh

[CMSSW_7_0_X] xrootd add XrdClient/XrdClientSid.hh private header