tshark: Add libxxhash to dependency list

tshark: Add libxxhash to dependency list

- From version 2.6.0 tshark added libxxhash as an option which is defined as ON by
   default. As libxxhash is built as a dependency for rsync and borgbackup the tshark
   build worked without problems but then the libxxhash library wass not present and so
   tshark failed to run.
- This patch adds libxxhash to the dependency list for tshark
- No change to rootfile

Signed-off-by: Adolf Belka <adolf.belka@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>