add osmo_gettimeofday as a shim around gettimeofday

This allows feeding a custom time for unit tests by overriding
osmo_gettimeofday.

Change-Id: Ic7a81a6eb51f27fe452962b91f2eae2070d87089
diff --git a/tests/gb/bssgp_fc_test.c b/tests/gb/bssgp_fc_test.c
index ad8f83d..d77f141 100644
--- a/tests/gb/bssgp_fc_test.c
+++ b/tests/gb/bssgp_fc_test.c
@@ -22,7 +22,7 @@
 {
 	struct timeval tv;
 	struct timeval now;
-	gettimeofday(&now, NULL);
+	osmo_gettimeofday(&now, NULL);
 
 	timersub(&now, &tv_start, &tv);
 
@@ -73,7 +73,7 @@
 	bssgp_fc_init(fc, bucket_size_max, bucket_leak_rate, max_queue_depth,
 		      fc_out_cb);
 
-	gettimeofday(&tv_start, NULL);
+	osmo_gettimeofday(&tv_start, NULL);
 
 	for (i = 0; i < pdu_count; i++) {
 		fc_in(fc, pdu_len);