ttcn3-tcpdump-stop.sh: compress capture files with gzip

Since we have enabled the GSMTAP logging, test case capture files
now contain a lot of textual information that can be efficiently
compressed with gzip.  That would make the "Build artifacts"
take much less space on Jenkins.

Change-Id: Ibdf5312bb9ff802d7ce7d00c690facdd79839782
1 file changed