Fix ppc compilation

/builddir/build/BUILD/xrootd-4.1.0/src/Xrd/XrdConfig.cc: In member function 'int XrdConfig::setFDL()':
/builddir/build/BUILD/xrootd-4.1.0/src/Xrd/XrdConfig.cc:831:17: error: comparison is always true due to limited range of data type [-Werror=type-limits]
    if (coreV >= 0)