Fix compiler warnings

jcc.c:1012:27: warning: implicit conversion changes signedness: 'jb_err' (aka 'enum privoxy_err') to 'int' [-Wsign-conversion]
         jb_err_to_string(err));
         ~~~~~~~~~~~~~~~~ ^~~
jcc.c:1721:45: warning: implicit conversion changes signedness: 'jb_err' (aka 'enum privoxy_err') to 'int' [-Wsign-conversion]
         csp->ip_addr_str, jb_err_to_string(err));
                           ~~~~~~~~~~~~~~~~ ^~~