fio: make iostat 5001 return FIO_ENDFL for gfortran.

In gfortran it seems that reading after the end of the file has been reached
returns 5001 (LIBERROR_OPTION_CONFLICT), rather than another 5008
(LIBERROR_ENDFILE) as for g95/g77. This is explicit in:

  generate_error (&fpp->common, LIBERROR_OPTION_CONFLICT,
                  "Cannot perform ENDFILE on a file already "
                  "positioned after the EOF marker");

We'll assume that 5001 is a 5008 from now on.