gettime.h: use time_t instead of size_t for copy of tv_sec

This addresses a small nit that our compiler was complaining about.
Thanks!

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/gettime.h b/gettime.h
index eb3537b..86d55bd 100644
--- a/gettime.h
+++ b/gettime.h
@@ -24,7 +24,7 @@
 
 static inline int fio_gettime_offload(struct timeval *tv)
 {
-	size_t last_sec;
+	time_t last_sec;
 
 	if (!fio_tv)
 		return 0;