dns/dnsdist: unbreak with boost 1.70

dnsdist-lua-vars.cc:99:61: error: expected unqualified-id

{"BADSIG",   ERCode::BADSIG  },
                     ^

/usr/include/sys/signal.h:490:17: note: expanded from macro 'BADSIG'
#define BADSIG SIG_ERR

^

/usr/include/sys/signal.h:139:18: note: expanded from macro 'SIG_ERR'
#define SIG_ERR ((__sighandler_t *)-1)

^

PR: 236587
Reported by: jbeich