libzpool: Set -Wno-error=typedef-redefinition for clang

In some versions of LLVM (at least 21), the <*intrin.h> headers contain
unguarded duplicate typedefs; this isn't permitted prior to C11, and
libzpool is built as C99. FreeBSD's LLVM backported LLVM PR #153820
to fix this, but other versions of LLVM (e.g., upstream, or on Linux)
don't have the patch, so this breaks the build.

Add -Wno-error=typedef-redefinition to downgrade this from an error
to a warning.

MFC after: 2 weeks
Reviewed by: dim, emaste
Sponsored by: https://www.patreon.com/bsdivy
Differential Revision: https://reviews.freebsd.org/D56653