FindHDF5: Add hdf5_hl to list of libraries to search for Fortran_HL

When the Fortran_HL component is specified, the hdf5_hl library is not included
in HDF5_LIBRARIES which causes a bunch of undefined references at link
time. This commit adds hdf5_hl to the list of libraries to search for when the
Fortran_HL component is specified.