<pthread.h>: Mark pthread_exit() as __noreturn

Same fix as https://android-review.googlesource.com/#/c/67540/
Fix for: https://code.google.com/p/android/issues/detail?id=60686

Change-Id: I27b8f1ed71dbb44f3598f7f4551bd881754ef945

ndk/platforms/android-3/include/pthread.h

@@ -138,7 +138,9 @@ int pthread_getattr_np(pthread_t thid, pthread_attr_t * attr);
 
 int pthread_create(pthread_t *thread, pthread_attr_t const * attr,
                    void *(*start_routine)(void *), void * arg);
-void pthread_exit(void * retval);
+
+__noreturn void pthread_exit(void * retval);
+
 int pthread_join(pthread_t thid, void ** ret_val);
 int pthread_detach(pthread_t  thid);