Core: Cleanup headers in core/config

- `MainLoop` now forward-declared in `OS`

drivers/apple_embedded/app_delegate_service.mm

@@ -35,6 +35,7 @@
 #import "os_apple_embedded.h"
 
 #include "core/config/project_settings.h"
+#include "core/os/main_loop.h"
 #import "drivers/coreaudio/audio_driver_coreaudio.h"
 #include "main/main.h"
 

drivers/apple_embedded/os_apple_embedded.mm

@@ -40,6 +40,7 @@
 #include "core/config/project_settings.h"
 #include "core/io/dir_access.h"
 #include "core/io/file_access.h"
+#include "core/os/main_loop.h"
 #import "drivers/apple/os_log_logger.h"
 #include "main/main.h"