Fix merging of enable_timestamps in configurations.

Without this, later interface definitions override earlier ones
even when they don't specify a value.

configuration.c

@@ -574,6 +574,7 @@ merge_ifconf(struct interface_conf *dest,
     MERGE(lq);
     MERGE(faraway);
     MERGE(channel);
+    MERGE(enable_timestamps);
     MERGE(rtt_exponential_decay);
     MERGE(rtt_min);
     MERGE(rtt_max);