During refactoring, the previously implemented NSSAI field of the cell
info structure was not written to F1AP ASN.1 structures anymore;
eq_f1ap_cell_info() also conveniently omitted the corresponding equality
check. Add this back so the CU can correctly receive the NSSAI in case
of F1 split operation.

Fixes: f278b008 ("Add encoding/decoding library for F1AP Setup
  Request (Interface Management) ")