Added DPP_EXPORT to bignum.h bn_deleter::operator()

[andrevanduin]

Added a missing DPP_EXPORT on bn_deleter::operator() since not exporting this symbol causes Windows build to fail since it does not export symbols by default.

Without the change during linking on Windows you get:

With the small change it works as expected.

Code change checklist

• I have ensured that all methods and functions are fully documented using doxygen style comments.
• My code follows the coding style guide.
• I tested that my change works before raising the PR.
• I have ensured that I did not break any existing API calls.
• I have not built my pull request using AI, a static analysis tool or similar without any human oversight.

[CLAassistant]

All committers have signed the CLA.

[github-actions]

Thank you for your contribution, but PRs must be raised against the dev branch. Please log your pull request against the dev branch not master. You can also retarget this pull request, then reopen it.