xrootd: add XrdClient/XrdClientProtocol.hh to private headers

CMSSW depends on XrdClient/XrdClientProtocol.hh private header.
Starting 3.3.3 header is no more deployed for public.

Signed-off-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>
Merge pull request #148 from davidlt/xrootd-add-xrdclientprotocol-hh

[CMSSW_7_0_X] XROOTD add XrdClient/XrdClientProtocol.hh to private headers