Bump cmake_minimum_required from 3.5 to 3.13
Match the minimum CMake version used by libheif.
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 673387b..cf4ebad 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -1,7 +1,7 @@
# Copyright 2019 Joe Drago. All rights reserved.
# SPDX-License-Identifier: BSD-2-Clause
-cmake_minimum_required(VERSION 3.5)
+cmake_minimum_required(VERSION 3.13)
# Specify search path for CMake modules to be loaded by include() and find_package()
list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/cmake/Modules")