MINOR: stats: display the number of threads in the statistics.

Add the nbthread global variable to the output, matching nbproc.

This may be backported to 1.8
diff --git a/include/types/stats.h b/include/types/stats.h
index 7022468..b412433 100644
--- a/include/types/stats.h
+++ b/include/types/stats.h
@@ -239,6 +239,7 @@
 	INF_NAME,
 	INF_VERSION,
 	INF_RELEASE_DATE,
+	INF_NBTHREAD,
 	INF_NBPROC,
 	INF_PROCESS_NUM,
 	INF_PID,