Contribs: Update Nettle to 2.7

This is required for gnutls 3.2.x+ building
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>
(cherry picked from commit 0baa1dbe8c3214dd6c312073438e1b3cf87e0d5e)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

contrib/src/nettle/SHA512SUMS

-edee4b7ecd34d7db8636650e59916c358a9be323128b4b7ad6eb4088a0a09a6744d0ebf382b23143fbcee3d40b148f43a54f25759141b16ba0befc3c46740b80  nettle-2.6.tar.gz
+394cbe567f958594e1bca42b48802e2fbc9c31c468cd1c399a0aec6f94138220b4bf3c307590262314664e15a74a7a52f9fd2b4f9fb01dde669e1fbe23e57caf  nettle-2.7.tar.gz

contrib/src/nettle/rules.mak

 # Nettle
 
-NETTLE_VERSION := 2.6
+NETTLE_VERSION := 2.7
 NETTLE_URL := ftp://ftp.gnu.org/gnu/nettle/nettle-$(NETTLE_VERSION).tar.gz
 
-ifeq ($(call need_pkg,"nettle >= 2.6"),)
+ifeq ($(call need_pkg,"nettle >= 2.7"),)
 PKGS_FOUND += nettle
 endif