ttcn3-tcpdump-start.sh: don't hardcode binary paths

After replacing debian-stretch-titan with debian-bullseye-titan, tcpdump
does not run anymore, the binary is in /usr/bin and not in /usr/sbin.

Related: OS#4969
Change-Id: I18c88c45c7522c1120ca42cd48d67fe8f155cd39
1 file changed