Update flesnet version

In the latest flesnet version a fix was added to avoid crashes due to a missing include statement. The error occors only with teh gcc8 compiler on Debian 8 and 10. refs #2161

Dear @f.uhlig,

you have been identified as code owner of at least one file which was changed with this merge request.

Please check the changes and approve them or request changes.

added CodeOwners label

merged