Support hash function from nettle (only)

Unlike COPTS=-DHAVE_DNSSEC, allow usage of just sha256 function from
nettle, but keep DNSSEC disabled at build time. Skips use of internal
hash implementation without support for validation built-in.

src/hash_questions.c

@@ -28,7 +28,7 @@
 
 #include "dnsmasq.h"
 
-#ifdef HAVE_DNSSEC
+#if defined(HAVE_DNSSEC) || defined(HAVE_NETTLEHASH)
 unsigned char *hash_questions(struct dns_header *header, size_t plen, char *name)
 {
   int q;