Fix the build in case FIO_HAVE_DISK_UTIL is not defined

This patch fixes the following build error:

stat.c: In function 'show_thread_status_terse_all':
stat.c:1243:31: error: suggest braces around empty body in an 'if' statement [-Werror=empty-body]
   show_disk_util(1, NULL, out);
                               ^

Signed-off-by: Bart Van Assche <[email protected]>

diskutil.h

@@ -120,7 +120,7 @@ static inline void print_disk_util(struct disk_util_stat *du,
 				   struct buf_output *out)
 {
 }
-#define show_disk_util(terse, parent, out)
+#define show_disk_util(terse, parent, out) do { } while (0)
 #define disk_util_prune_entries()
 #define init_disk_util(td)
 #define setup_disk_util()