Fix build with LibreSSL
Differential Revision: https://reviews.FreeBSD.org/D9334
Submitted by: brnrd
Obtained from: https://github.com/nghttp2/nghttp2/commit/16be89f9ccba4d37e34ef03f20009a9845efeb99
https://github.com/nghttp2/nghttp2/commit/1cc08c0a51d273afc0e7b4e11eeb0b889d154ee0