math/givaro: update 4.2.0 -> 4.2.1, fix build with libc++ 21

• libc++ 21 no longer allows std::make_signed to be overridden. Upstream fixed this as part of https://github.com/linbox-team/givaro/commit/ed91ee0

PR: 293269
Changes: https://github.com/linbox-team/givaro/releases/tag/v4.2.1