usrsctp: fix build with clang 15+

[reckenrode]

Description of changes

• Add missing function prototypes; and
• Remove set but unused variables.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 23.11 Release Notes (or backporting 23.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

[misuzu]

I don't think it's a good idea to update this library to some random commit. Maybe there's a way to fix this clang16 issue while still using a tagged release.
Also this change should go to the staging

[reckenrode]

I don't think it's a good idea to update this library to some random commit.

Ideally, but that letter assumes projects doing regular releases, which is not the case here. This one doesn’t. It only has three releases, and the last release was very minor (just cleaning up .so versioning). In the issue requesting what would become the 0.9.4.0 release, it was questioned why people couldn’t just use snapshots instead. The release notes for that version are simply, “Current status as requested in #561.”

Maybe there's a way to fix this clang16 issue while still using a tagged release. Also this change should go to the staging

Sure, I can write and vendor a patch in nixpkgs. I normally like the to cherry-pick commits from upstream that fix the issue, but those here are mixed in with changes other than just the fixes. I rebased my staging branch last night, so it’s going to be a little while before I have a patch, but I should be able to update this PR by this evening EDT.

[reckenrode]

I updated the PR with a patch to remove the unused variables and add the missing prototypes. I initially tried applying the commits from upstream FreeBSD that did this, but they didn’t apply cleanly, so I made the changes manually to create the included patch. I also retargeted to staging.