Fix special flags enum
Fix special flags enum

I accidentally allowed overlapping enum values in #6977d68. One symptom
is that parsing "-0.0" sets only the JSONSL_SPECIALf_UNSIGNED flag, not
JSONSL_SPECIALf_FLOAT|JSONSL_SPECIALf_SIGNED as it should.

mnunberg/jsonsl,snej/jsonsl,snej/jsonsl,mnunberg/jsonsl,snej/jsonsl,mnunberg/jsonsl,snej/jsonsl,mnunberg/jsonsl