meson: fix building with -Ddns-over-tls=false

Assigning multiple variables in one line is no valid meson syntax.
Without this running meson with -Ddns-over-tls=false fails with:

meson.build:1191:8: ERROR:  Tried to assign an invalid value to variable.

meson.build

@@ -1188,7 +1188,9 @@ if dns_over_tls != 'false'
         endif
         have = have_gnutls or have_openssl
 else
-        have = have_gnutls = have_openssl = false
+        have = false
+        have_gnutls = false
+        have_openssl = false
 endif
 conf.set10('ENABLE_DNS_OVER_TLS', have)
 conf.set10('DNS_OVER_TLS_USE_GNUTLS', have_gnutls)