pthreadx: add pthread_mutexattr_destroy to header
Fixes warning on MSVC build:
openwsman\src\lib\u\lock.c(64): warning C4013: 'pthread_mutexattr_destroy' undefined; assuming extern returning int
@@ -68,6 +68,7 @@ extern int pthread_mutex_destroy(pthread_mutex_t *mp);
extern int pthread_mutexattr_settype(pthread_mutexattr_t *attr, int t);
extern int pthread_mutex_trylock(pthread_mutex_t *m);
extern int pthread_mutexattr_init(pthread_mutexattr_t *attr);
+extern int pthread_mutexattr_destroy(pthread_mutexattr_t *attr);
#endif /* WIN32 */
