[mod_vhostdb_mysql] MySQL missing mysql_get_socket (fixes #3208)

MySQL does not have mysql_get_socket(); MariaDB has mysql_get_socket()

x-ref:
  "Lighttpd 1.4.70's `src/mod_vhostdb_mysql.c` change is incompatible with MySQL 8"
  https://redmine.lighttpd.net/issues/3208

src/mod_vhostdb_mysql.c

@@ -114,7 +114,11 @@ static int mod_vhostdb_dbconf_setup (server *srv, const array *opts, void **vdat
             return -1;
         }
 
+      #ifdef LIBMARIADB
         my_socket sfd = mysql_get_socket(dbconn);
+      #else
+        my_socket sfd = dbconn->net.fd;
+      #endif
         (void)fdevent_socket_set_cloexec(sfd);
 
         dbconf = (vhostdb_config *)ck_calloc(1, sizeof(*dbconf));