fixup ttcn3-tcpdump-stop.sh: add flag '--force' to gzip

When a directory path for the "build artifacts" is not specified,
'/tmp' would be used by default.  When running the same text
case more than once, gzip will be asking for confirmation whether
to overwrite the existing capture file.  Let's do this by default.

Change-Id: I357f3d9c5dc5963f4b709166bb659f3f66a721b8
diff --git a/ttcn3-tcpdump-stop.sh b/ttcn3-tcpdump-stop.sh
index ac192ca..b2a3a3e 100755
--- a/ttcn3-tcpdump-stop.sh
+++ b/ttcn3-tcpdump-stop.sh
@@ -49,4 +49,4 @@
 kill_rm_pidfile "$PIDFILE_PCAP"
 kill_rm_pidfile "$PIDFILE_NETCAT"
 
-gzip "$TTCN3_PCAP_PATH/$TESTCASE.pcap"
+gzip -f "$TTCN3_PCAP_PATH/$TESTCASE.pcap"