Made json_value::clone() private.
Made json_value::clone() private.

json_value::clone() returns a newed raw pointer,
so it is dangerous to expose to users.

pfi/pficommon,pfi/pficommon,takei-yuya/pficommon,pfi/pficommon,retrieva/pficommon,takei-yuya/pficommon,retrieva/pficommon,pfi/pficommon,retrieva/pficommon,retrieva/pficommon,retrieva/pficommon,takei-yuya/pficommon,takei-yuya/pficommon