Forward-declare OQS_SIG in sig_stfl.h

[SWilson4]

When an application (e.g., oqs-provider) includes <oqs/sig.h>, the following happens:

• sig.h includes oqs.h at the top of the file.
• oqs.h includes sig.h, but this include is skipped because of the OQS_SIG_H include guard.
• oqs.h includes sig_stfl.h.
• sig_stfl.h references OQS_SIG, which has not yet been defined.

This breaks the application's build: see https://github.com/open-quantum-safe/liboqs/issues/1815, https://github.com/open-quantum-safe/oqs-provider/issues/427, and https://github.com/open-quantum-safe/oqs-demos/issues/281.

This PR resolves the issue by forward-declaring the OQS_SIG type in sig_stfl.h. This fixes the failing oqs-provider build: see the downstream tests before and after.

I don't believe that the forward-declaration violates any C programming rules / best practices, but it would be great if someone with more experience could confirm this.

• [ ] Does this PR change the input/output behaviour of a cryptographic algorithm (i.e., does it change known answer test values)? (If so, a version bump will be required from x.y.z to x.(y+1).0.)
• [ ] Does this PR change the list of algorithms available -- either adding, removing, or renaming? Does this PR otherwise change an API? (If so, PRs in fully supported downstream projects dependent on these, i.e., oqs-provider will also need to be ready for review and merge by the time this is merged.)

[ashman-p]

@SWilson4 Thanks for looking at this and resolving it. I did also find that an alternative resolution but i prefer this fix.