added guard to prevent re-defining names in C23 - #68

> gcc tinycthread.c -c
In file included from tinycthread.c:25:
tinycthread.h:460:21: error: conflicting types for ‘pthread_once_t’; have ‘__once_flag’
  460 |   #define once_flag pthread_once_t
      |                     ^~~~~~~~~~~~~~
In file included from /usr/include/pthread.h:26,
                 from tinycthread.h:87:
/usr/include/x86_64-linux-gnu/bits/pthreadtypes.h:53:30: note: previous declaration of ‘pthread_once_t’ with type ‘pthread_once_t’ {aka ‘int’}
   53 | typedef int __ONCE_ALIGNMENT pthread_once_t;
      |                              ^~~~~~~~~~~~~~
tinycthread.h:472:32: error: conflicting types for ‘pthread_once’; have ‘void(int *, void (*)(void))’
  472 |   #define call_once(flag,func) pthread_once(flag,func)
      |                                ^~~~~~~~~~~~
/usr/include/pthread.h:509:12: note: previous declaration of ‘pthread_once’ with type ‘int(pthread_once_t *, void (*)(void))’ {aka ‘int(int *, void (*)(void))’}
  509 | extern int pthread_once (pthread_once_t *__once_control,
      |            ^~~~~~~~~~~~