ROOT: Add xrootd as a recommended dependency

xrootd is one of the primary transport protocols that ROOT supports, and
is widely used at CERN and on the LHC Computing Grid.  Currently, for
ROOT to compile support for xrootd, the user must have it installed
already.  This commit adds a recommended xrootd dependency so that users
will automatically have support for this important data-access component
of ROOT with minimal overhead (xrootd itself not an enormous dependency
and has compile time much less than that of ROOT itself).