Update nettle package with info for v3.4 (#8653)

var/spack/repos/builtin/packages/nettle/package.py

@@ -32,6 +32,7 @@ class Nettle(AutotoolsPackage):
     homepage = "https://www.lysator.liu.se/~nisse/nettle/"
     url      = "http://ftp.gnu.org/gnu/nettle/nettle-3.3.tar.gz"
 
+    version('3.4',   'dc0f13028264992f58e67b4e8915f53d')
     version('3.3',   '10f969f78a463704ae73529978148dbe')
     version('3.2',   'afb15b4764ebf1b4e6d06c62bd4d29e4')
     version('2.7.1', '003d5147911317931dd453520eb234a5')