Fix double + in hkdflabel declaration (FIXES #10675)
CLA: trivial
Reviewed-by: Matt Caswell <matt@openssl.org> Reviewed-by: Kurt Roeckx <kurt@roeckx.be> Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
(Merged from https://github.com/openssl/openssl/pull/10700)