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")