unified2: fix logging of tagged packets

The structure for create the alert preceding each tagged packet
was not being initialized, preventing tagged packets from being
logged.

Note: Snort unified2 does not precede tagged packets with an
alert like is done here, so this just fixes what the code
intended to do, it does not make it Snort unified2
compatible.

Address issue:
https://redmine.openinfosecfoundation.org/issues/1854

src/suricata.c

@@ -2376,6 +2376,7 @@ static int PostConfLoadedSetup(SCInstance *suri)
     SCProtoNameInit();
 
     TagInitCtx();
+    PacketAlertTagInit();
     ThresholdInit();
     HostBitInitCtx();
     IPPairBitInitCtx();