contrib: update nettle

Change-Id: Iae41a073cfa5a1a842d8025477539db833f506b8

contrib/src/nettle/SHA512SUMS

-f738121b9091cbe79435fb5d46b45cf6f10912320c233829356908127bab1cac6946ca56e022a832380c44f2c10f21d2feef64cb0f4f41e3da4a681dc0131784  nettle-3.5.1.tar.gz
\ No newline at end of file
+2471af875e51327af61af8bda53cd9c3adc27b6e32592a4b5b10b3ec60999ebf771ab9c54c747b0bade4b3b5a717e77fdbdb53699dd9e8a9ed4eee07f46aed51  nettle-3.6.tar.gz

contrib/src/nettle/rules.mak

 # Nettle
 
-NETTLE_VERSION := 3.5.1
+NETTLE_VERSION := 3.6
 NETTLE_URL := $(GNU)/nettle/nettle-$(NETTLE_VERSION).tar.gz
 
 PKGS += nettle