condor_scitokens: fix return type for scitoken_free_string_list
condor_scitokens: fix return type for scitoken_free_string_list

this should be `void`, ref:
https://github.com/scitokens/scitokens-cpp/blob/v0.6.2/src/scitokens.cpp#L160-L166

htcondor/htcondor,htcondor/htcondor,htcondor/htcondor,htcondor/htcondor,htcondor/htcondor,htcondor/htcondor,htcondor/htcondor,htcondor/htcondor