Missing PRCELLSTATE declarations in some mod files

[iomaganaris]

For some mod files like https://github.com/BlueBrain/CoreNeuron/blob/master/coreneuron/mechanism/mech/modfile/passive.mod the generated c++ code has an empty nrn_state function that doesn't have the PRCELLSTATE macro to update the v_unused variable of the mechanism similar to NMODL.
This has as a consequence on the prcellstate the v_unused variable for this mechanism to be different between mod2c and NMODL.
Instead of not printing anything in the nrn_state function, if the CORENRN_ENABLE_PRCELLSTATE CMake variable is set, modc should print the code needed to update the v_unused variable.