cmake: bump minimum required to support example

The sub-directory "example" takes advantage of the command
"target_include_directories", which is only available in CMake
v2.8.11+ [1][2]. Bumping the minimum version of CMake required.

[1]: https://cmake.org/cmake/help/v2.8.11/cmake.html#command:target_include_directories
[2]: https://cmake.org/cmake/help/v2.8.10/cmake.html#section_Commands

Signed-off-by: James Knight <james.d.knight@live.com>
diff --git a/CMakeLists.txt b/CMakeLists.txt
index a5bff17..be7d88a 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -28,7 +28,7 @@
 # The SPIR-V headers from the SPIR-V Registry
 # https://www.khronos.org/registry/spir-v/
 #
-cmake_minimum_required(VERSION 2.8)
+cmake_minimum_required(VERSION 2.8.11)
 project(SPIRV-Headers)
 
 # There are two ways to use this project.