Fix CI build

[abeimler]

Build Error:

In file included from /home/runner/work/ecs_benchmark/ecs_benchmark/build/_deps/mustache-src/src/mustache/ecs/system.cpp:1:
Warning: /home/runner/work/ecs_benchmark/ecs_benchmark/build/_deps/mustache-src/src/mustache/ecs/system.hpp:27:10: warning: elaborated-type-specifier for a scoped enum must not use the ‘class’ keyword
   27 |     enum class SystemState : uint32_t {
      |     ~~~~ ^~~~~
      |          -----
Error: /home/runner/work/ecs_benchmark/ecs_benchmark/build/_deps/mustache-src/src/mustache/ecs/system.hpp:27:28: error: found ‘:’ in nested-name-specifier, expected ‘::’
   27 |     enum class SystemState : uint32_t {
      |                            ^
      |                            ::

Current Workaround

Add #include <cstdint> in mustache-src/src/mustache/ecs/system.hpp:

system.hpp

#pragma once

#include <cstdint>
#include <mustache/utils/uncopiable.hpp>
#include <mustache/utils/type_info.hpp>

#include <set>