dns: use ARES_LIB_INIT_NONE instead of a magic number

the parameter passed to `ares_library_init()` is a bitmap. so 0
is indeed nothing but `ARES_LIB_INIT_NONE` is more readable.

Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>

src/net/dns.cc

@@ -124,7 +124,7 @@ static void check_ares_error(int error) {
 
 struct ares_initializer {
     ares_initializer() {
-        check_ares_error(ares_library_init(0));
+        check_ares_error(ares_library_init(ARES_LIB_INIT_NONE));
     }
     ~ares_initializer() {
         ares_library_cleanup();