Vk: require at least CMake 3.7.

Older versions don't have FindVulkan and I can't be bothered.

src/Magnum/Vk/CMakeLists.txt

@@ -24,6 +24,9 @@
 #   DEALINGS IN THE SOFTWARE.
 #
 
+# CMake has FindVulkan since 3.7
+cmake_minimum_required(VERSION 3.7)
+
 # We don't need system-wide includes
 set(Vulkan_INCLUDE_DIR ${PROJECT_SOURCE_DIR}/src/MagnumExternal/Vulkan)
 find_package(Vulkan REQUIRED)