Add versioning information to symbols.

Bug: http://b/28178111
Change-Id: I46bf95accd819f4521afb1173d8badcc5e9df31c
diff --git a/libc/include/sys/epoll.h b/libc/include/sys/epoll.h
index 4a5a37c..4ec8969 100644
--- a/libc/include/sys/epoll.h
+++ b/libc/include/sys/epoll.h
@@ -74,10 +74,10 @@
 ;
 
 int epoll_create(int);
-int epoll_create1(int);
+int epoll_create1(int) __INTRODUCED_IN(21);
 int epoll_ctl(int, int, int, struct epoll_event*);
 int epoll_wait(int, struct epoll_event*, int, int);
-int epoll_pwait(int, struct epoll_event*, int, int, const sigset_t*);
+int epoll_pwait(int, struct epoll_event*, int, int, const sigset_t*) __INTRODUCED_IN(21);
 
 __END_DECLS