gprs_rlcmac_received_lost(): Fix regression / uninitialized now_tv

In Change-Id I7d22e7b5902c230efeae66eb20c17026a4037887 we
introduced the use of timespecsub(). Unfortuantely, we also
accidentially removed the call to osmo_clock_gettime() along
with it, leaving now_tv completely uninitialized.

Change-Id: Ieced0c62700b2fe4ab0208258183154cc701490b
Related: OS#3225
Fixes: Coverity CID#188872
diff --git a/src/gprs_rlcmac_meas.cpp b/src/gprs_rlcmac_meas.cpp
index f6448d5..9ff06db 100644
--- a/src/gprs_rlcmac_meas.cpp
+++ b/src/gprs_rlcmac_meas.cpp
@@ -131,6 +131,7 @@
 	tbf->m_bw.dl_loss_received += received;
 	tbf->m_bw.dl_loss_lost += lost;
 
+	osmo_clock_gettime(CLOCK_MONOTONIC, &now_tv);
 	timespecsub(&now_tv, loss_tv, &elapsed);
 	if (elapsed.tv_sec < 1)
 		return 0;