Fixes a C89 pedantic compliance error in src/modules/ellswift/tests_impl.h where function pointer array initialization is not allowed at declaration time.
This error was exposed while I was testing the improved test coverage in CI. The initial plan was to simplify the configuration of modules in CI by enabling all modules by default and testing the disabling of each module independently.
Error: src/modules/ellswift/tests_impl.h:442:110: error: initializer element is not computable at load time [-Wpedantic].
The error occurred when running the x86_64_debian GitHub Actions CI job, which uses GCC 16 (snapshot) with strict flags (-std=c89 -pedantic -pedantic-errors -Werror). See this action run for reference: https://github.com/mllwchrry/secp256k1/actions/runs/23301905657/job/67769464566.
The fix uses if/else to assign function pointers after declaration, matching the pattern already used in the same file.
While this is a minor C89 compliance issue, it blocks the potential CI simplification.
