zephyr: hello-world: add cmake_minimum_required

The sample fails to build without it now due to changes in the
boilerplate file.

Signed-off-by: Marti Bolivar <marti@foundries.io>
diff --git a/samples/zephyr/hello-world/CMakeLists.txt b/samples/zephyr/hello-world/CMakeLists.txt
index 76275e8..3dae664 100644
--- a/samples/zephyr/hello-world/CMakeLists.txt
+++ b/samples/zephyr/hello-world/CMakeLists.txt
@@ -1,6 +1,7 @@
 # Top-level CMakeLists.txt for the skeleton application.
 #
 # Copyright (c) 2017 Open Source Foundries Limited
+# Copyright (c) 2018 Foundries.io Ltd
 #
 # SPDX-License-Identifier: Apache-2.0
 #
@@ -8,6 +9,8 @@
 # mcuboot, which is easy to customize on a per-board basis. It can be
 # used as a starting point for new applications.
 
+cmake_minimum_required(VERSION 3.8)
+
 # The default top-level application configuration is prj.conf.
 # You can place additional board-specific files in boards/${BOARD}.conf,
 # and they will be merged into the configuration along with prj.conf.