Contribs: update Nettle to 2.7.1

(cherry picked from commit dc2a22c095453c40657f5ac83a289740beceb30e)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

contrib/src/nettle/SHA512SUMS

-394cbe567f958594e1bca42b48802e2fbc9c31c468cd1c399a0aec6f94138220b4bf3c307590262314664e15a74a7a52f9fd2b4f9fb01dde669e1fbe23e57caf  nettle-2.7.tar.gz
+297c69e90bbd448f72e854abe5cc7868c08d710e1c1bcd6a14adf06e25629d58a3ef4d65ab588d001ec7091aa583032312ad15b416ea5479e5bf0ea63717f473  nettle-2.7.1.tar.gz

contrib/src/nettle/rules.mak

 # Nettle
 
-NETTLE_VERSION := 2.7
+NETTLE_VERSION := 2.7.1
 NETTLE_URL := ftp://ftp.gnu.org/gnu/nettle/nettle-$(NETTLE_VERSION).tar.gz
 
 ifeq ($(call need_pkg,"nettle >= 2.7"),)