Looking through the changelogs and the various files from the wireshark website, I have been unable to find anything that says that libxxhash or xxhash is a required dependency for build or for runtime.
libxxhash is available on IPFire as it is a required dependency for borgbackup and for rsync and it is built before tshark is built so it would not cause a problem for building tshark because it would be available in the build system.
It is very strange that they have made it into a build and run requirement and I have checked and it is not referenced in any of the change logs from 4.4.6 to 4.6.2
I have checked what Arch Linux are doing and they do have it defined as both a build and runtime requirement.
As a workaround, if you install rsync it will also install libxxhash and then tshark will run.
I will create a patch update to add libxxhash as a dependency to tshark.
I have found that it has been added in as an option into the CMAKE Options file from version 2.6.0 and the default status has been set to ON so that the option is required. The same is done for zlib and other libraries but all of those are used in the core packages of IPFire and are therefore available for both build and runtime.
Although added as default ON option in version 2.6.0 it is not mentioned in the 2.6.0 changelog which only references another cmake option that has been made deprecated in version 2.6.0