Refactor header includes in athr.h

- Removed platform-specific include guards for improved portability.
- Unified `#include <stdatomic.h>` and `#include <stdbool.h>` usage.
- Simplified the header structure by maintaining essential includes.

This change enhances the maintainability by eliminating redundant and
conditional includes. Keeping the header cleaner makes it easier to update
and less error-prone when expanding cross-platform support. 🚀

athr.h

@@ -5,15 +5,9 @@
 #include "athr_option.h"
 #include "athr_thread.h"
 #include "athr_widget_main.h"
-#include <stdint.h>
-
-#if defined(ATHR_OS_WIN32)
-#include "athr_ovs_atomic_msvc.h"
-#elif defined(ATHR_OS_UNIX)
 #include <stdatomic.h>
 #include <stdbool.h>
 #include <stdint.h>
-#endif
 
 /* How often to update initially, in milliseconds. */
 #define ATHR_TIMESTEP 250