Use CLOCK_MONOTONIC if we can

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/gettime.c b/gettime.c
index 72fda3f..15356b1 100644
--- a/gettime.c
+++ b/gettime.c
@@ -140,7 +140,11 @@
 	case CS_CGETTIME: {
 		struct timespec ts;
 
+#ifdef FIO_HAVE_CLOCK_MONOTONIC
+		if (clock_gettime(CLOCK_MONOTONIC, &ts) < 0) {
+#else
 		if (clock_gettime(CLOCK_REALTIME, &ts) < 0) {
+#endif
 			log_err("fio: clock_gettime fails\n");
 			assert(0);
 		}