Added integer flag to ecl_file_open: To allign with ert commit: bd542a45

I have added a integer flag to the ert function ecl_file_open(). This was added in ert commit: 

https://github.com/Ensembles/ert/commit/bd542a45c5e72e94cbe79c8b9dd17feeace4e7b5

This PR adjusts opm-core align with the ert change.