Support for Mac OS 10.6.2

Contributed by Small Tree, adopted to newer fio version by me.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/helpers.h b/helpers.h
index a3a773b..61a3f2b 100644
--- a/helpers.h
+++ b/helpers.h
@@ -6,5 +6,6 @@
 extern int __weak posix_memalign(void **ptr, size_t align, size_t size);
 extern int __weak posix_fallocate(int fd, off_t offset, off_t len);
 extern int __weak inet_aton(const char *cp, struct in_addr *inp);
+extern int __weak clock_gettime(clockid_t clk_id, struct timespec *ts);
 
 #endif