make inadvertently exposed __pthread_{timed,try}join_np functions static

these exist for the sake of defining the corresponding weak public
aliases (for C11 and POSIX namespace conformance reasons). they are
not referenced by anything else in libc, so make them static.

src/thread/pthread_join.c

@@ -5,7 +5,7 @@ int __munmap(void *, size_t);
 void __pthread_testcancel(void);
 int __pthread_setcancelstate(int, int *);
 
-int __pthread_timedjoin_np(pthread_t t, void **res, const struct timespec *at)
+static int __pthread_timedjoin_np(pthread_t t, void **res, const struct timespec *at)
 {
 	int state, cs, r = 0;
 	__pthread_testcancel();
@@ -28,7 +28,7 @@ int __pthread_join(pthread_t t, void **res)
 	return __pthread_timedjoin_np(t, res, 0);
 }
 
-int __pthread_tryjoin_np(pthread_t t, void **res)
+static int __pthread_tryjoin_np(pthread_t t, void **res)
 {
 	return t->detach_state==DT_JOINABLE ? EBUSY : __pthread_join(t, res);
 }