Wrong name for preprocessor variable since merge of PR #3219

**Describe the bug**

In [multiphysics/HydrofractureSolver.cpp:1052](https://github.com/GEOS-DEV/GEOS/blob/9cf5ab5540ea296194363713f1bb3972dc618d53/src/coreComponents/physicsSolvers/multiphysics/HydrofractureSolver.cpp#L1052), the GEOSX_USE_SEPARATION_COEFFICIENT preproc variable is tested while it has be renamed as GEOS_USE... by PR #3219. 


