Fix header path for experimental.h in build.rs

[K1ngfish3r]

Should Close #1275

When libgit2-1.9.4 is built with -DEXPERIMENTAL_SHA256=ON it sets the header files in the following directory

$ ls /usr/include/git2-experimental
annotated_commit.h  clone.h               email.h         merge.h        patch.h       reset.h      sys/
apply.h             commit.h              errors.h        message.h      pathspec.h    revert.h     tag.h
attr.h              common.h              experimental.h  net.h          proxy.h       revparse.h   trace.h
blame.h             config.h              filter.h        notes.h        rebase.h      revwalk.h    transaction.h
blob.h              cred_helpers.h        global.h        object.h       refdb.h       signature.h  transport.h
branch.h            credential.h          graph.h         odb.h          reflog.h      stash.h      tree.h
buffer.h            credential_helpers.h  ignore.h        odb_backend.h  refs.h        status.h     types.h
cert.h              deprecated.h          index.h         oid.h          refspec.h     stdint.h     version.h
checkout.h          describe.h            indexer.h       oidarray.h     remote.h      strarray.h   worktree.h
cherrypick.h        diff.h                mailmap.h       pack.h         repository.h  submodule.h

relevant libgit2
https://github.com/libgit2/libgit2/blob/main/cmake/ExperimentalFeatures.cmake#L21-L23
https://github.com/libgit2/libgit2/blob/main/src/libgit2/CMakeLists.txt#L122