Use const for some IPv6 structs used as fn args

[utoni]

That'll fix warning i.e.:

/home/toni/git/nDPId/nDPId.c: In function ‘ndpi_process_packet’:
/home/toni/git/nDPId/nDPId.c:4443:39: warning: passing argument 2 of ‘ndpi_flowv6_flow_hash’ discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
 4443 |                                       &ip6->ip6_src,
      |                                       ^~~~~~~~~~~~~

Please sign (check) the below before submitting the Pull Request:

• I have signed the ntop Contributor License Agreement at https://github.com/ntop/legal/blob/main/individual-contributor-licence-agreement.md
• I have read the contributing guidelines at https://github.com/ntop/nDPI/blob/dev/CONTRIBUTING.md
• I have updated the documentation (in doc/) to reflect the changes made (if applicable)

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[IvanNardi]

Thanks!