MINOR: threads/debug: only report used lock stats

The lock stats are very verbose and more than half of them are used in
a typical test, making it hard to spot the sought values. Let's simply
report "not used" for those which have not been called at all.
diff --git a/include/haproxy/thread.h b/include/haproxy/thread.h
index 810eb6f..8457121 100644
--- a/include/haproxy/thread.h
+++ b/include/haproxy/thread.h
@@ -454,6 +454,15 @@
 	int lbl;
 
 	for (lbl = 0; lbl < LOCK_LABELS; lbl++) {
+		if (!lock_stats[lbl].num_write_locked &&
+		    !lock_stats[lbl].num_seek_locked &&
+		    !lock_stats[lbl].num_read_locked) {
+			fprintf(stderr,
+			        "Stats about Lock %s: not used\n",
+			        lock_label(lbl));
+			continue;
+		}
+
 		fprintf(stderr,
 			"Stats about Lock %s: \n"
 			"\t # write lock  : %lu\n"