Contribs: update GnuTLS to 3.2.18

(cherry picked from commit b63dc639c0e566cfe3b5b941601e060c6d24f501)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

contrib/src/gnutls/SHA512SUMS

-275a13bb705539ae402269c9b67aa7f78bf57892f8b24727a2529de6024454fcbc4fd1ed9922120b33c30dad964a1f473f5222243b35eda7010538e7e3b61bef  gnutls-3.2.17.tar.xz
+fc364be7997d31e1a1855d871b4a9564ee5a3e9c2900839f9c32fcc656e67c916ef634508bc0363b8b3f0cba131c97596509f838c1e9b3ac2b56b224eee43908  gnutls-3.2.18.tar.xz

contrib/src/gnutls/rules.mak

 # GnuTLS
 
-GNUTLS_VERSION := 3.2.17
+GNUTLS_VERSION := 3.2.18
 GNUTLS_URL := ftp://ftp.gnutls.org/gcrypt/gnutls/v3.2/gnutls-$(GNUTLS_VERSION).tar.xz
 
 ifdef BUILD_NETWORK