Add preferences help tooltips

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/debug.h b/debug.h
index af71d62..b55a1e4 100644
--- a/debug.h
+++ b/debug.h
@@ -27,6 +27,7 @@
 #ifdef FIO_INC_DEBUG
 struct debug_level {
 	const char *name;
+	const char *help;
 	unsigned long shift;
 	unsigned int jobno;
 };