Fix pthread_barrierattr_getpshared prototype.

As http://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_barrierattr_getpshared.html
is my witness, there's a missing `const` here.

Change-Id: Ifd68188fb952460b64d144bdc21a53bc40ed82a7
diff --git a/libc/bionic/pthread_barrier.cpp b/libc/bionic/pthread_barrier.cpp
index 1bcd12a..1618222 100644
--- a/libc/bionic/pthread_barrier.cpp
+++ b/libc/bionic/pthread_barrier.cpp
@@ -42,7 +42,7 @@
   return 0;
 }
 
-int pthread_barrierattr_getpshared(pthread_barrierattr_t* attr, int* pshared) {
+int pthread_barrierattr_getpshared(const pthread_barrierattr_t* attr, int* pshared) {
   *pshared = (*attr & 1) ? PTHREAD_PROCESS_SHARED : PTHREAD_PROCESS_PRIVATE;
   return 0;
 }
diff --git a/libc/include/pthread.h b/libc/include/pthread.h
index 1153695..47dc153 100644
--- a/libc/include/pthread.h
+++ b/libc/include/pthread.h
@@ -232,8 +232,8 @@
 
 int pthread_barrierattr_init(pthread_barrierattr_t* _Nonnull attr) __INTRODUCED_IN(24);
 int pthread_barrierattr_destroy(pthread_barrierattr_t* _Nonnull attr) __INTRODUCED_IN(24);
-int pthread_barrierattr_getpshared(pthread_barrierattr_t* _Nonnull attr, int* _Nonnull pshared)
-  __INTRODUCED_IN(24);
+int pthread_barrierattr_getpshared(const pthread_barrierattr_t* _Nonnull attr,
+                                   int* _Nonnull pshared) __INTRODUCED_IN(24);
 int pthread_barrierattr_setpshared(pthread_barrierattr_t* _Nonnull attr, int pshared)
   __INTRODUCED_IN(24);