gettime: use unsigned loop counter

We're comparing with an unsigned exit condition.

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/gettime.c b/gettime.c
index c6d45f8..98e83b8 100644
--- a/gettime.c
+++ b/gettime.c
@@ -537,7 +537,7 @@
 	unsigned long tentries, failed;
 	struct clock_entry *prev, *this;
 	uint32_t seq = 0;
-	int i;
+	unsigned int i;
 
 	log_info("cs: reliable_tsc: %s\n", tsc_reliable ? "yes" : "no");