Contribs: update gnutls to 3.2.21

(cherry picked from commit 52088d39d1bf4bcfc19ee5a4ff2ec6633304d2cb)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

contrib/src/gnutls/SHA512SUMS

-fc364be7997d31e1a1855d871b4a9564ee5a3e9c2900839f9c32fcc656e67c916ef634508bc0363b8b3f0cba131c97596509f838c1e9b3ac2b56b224eee43908  gnutls-3.2.18.tar.xz
+8f89ac887d6bbe9ffec43a5a5e576daca563f3c1a7806e9a23963ef947b121b1d9562ee624a0bc427eda92652e24c83700fbfca283f29bbcf399ebcfe1fc218b  gnutls-3.2.21.tar.xz

contrib/src/gnutls/rules.mak

 # GnuTLS
 
-GNUTLS_VERSION := 3.2.18
+GNUTLS_VERSION := 3.2.21
 GNUTLS_URL := ftp://ftp.gnutls.org/gcrypt/gnutls/v3.2/gnutls-$(GNUTLS_VERSION).tar.xz
 
 ifdef BUILD_NETWORK