meson: do not compare objects of different types

This fixes the following warning:
```
meson.build:1140: WARNING: Trying to compare values of different types (DependencyHolder, list) using !=.
The result of this is undefined and will become a hard error in a future Meson release.
```

Follow-up for f02582f(#9410).

meson.build

@@ -1137,7 +1137,8 @@ substs.set('DEFAULT_DNSSEC_MODE', default_dnssec)
 
 dns_over_tls = get_option('dns-over-tls')
 if dns_over_tls != 'false'
-        have = libgnutls != [] and libgnutls.version().version_compare('>=3.5.3')
+        have = (conf.get('HAVE_GNUTLS') == 1 and
+                libgnutls.version().version_compare('>=3.5.3'))
         if dns_over_tls == 'true' and not have
                 error('DNS-over-TLS support was requested, but dependencies are not available')
         endif