Change cmake required to v3.13

This is because target_link_options was added in v3.13 So this wouldn't build with cmake v3.12 https://cmake.org/cmake/help/latest/release/3.13.html#commands

Author: ytnuf

CMakeLists.txt

@@ -37,7 +37,7 @@
 # Todo
 # Test build for Windows, Mac and mingw.
 
-cmake_minimum_required(VERSION 3.12)
+cmake_minimum_required(VERSION 3.13)
 project(godot-cpp LANGUAGES CXX)
 
 option(GENERATE_TEMPLATE_GET_NODE "Generate a template version of the Node class's get_node." ON)