gnutls: fix compilation with 3.3.0+
gnutls: fix compilation with 3.3.0+

(cherry picked from commit b5a7662a2c735ecb22b23f60a91c4f9c9162f3b8)

jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2