Implement pthread barrier.

Bug: 24341262
Change-Id: I5472549e5d7545c1c3f0bef78235f545557b9630
diff --git a/libc/Android.mk b/libc/Android.mk
index 2ea96d4..a7fe15c 100644
--- a/libc/Android.mk
+++ b/libc/Android.mk
@@ -563,6 +563,7 @@
 libc_pthread_src_files := \
     bionic/pthread_atfork.cpp \
     bionic/pthread_attr.cpp \
+    bionic/pthread_barrier.cpp \
     bionic/pthread_cond.cpp \
     bionic/pthread_create.cpp \
     bionic/pthread_detach.cpp \