Add versioning information to symbols.

Bug: http://b/28178111
Change-Id: I46bf95accd819f4521afb1173d8badcc5e9df31c
diff --git a/libc/include/pty.h b/libc/include/pty.h
index bca1137..ec257a5 100644
--- a/libc/include/pty.h
+++ b/libc/include/pty.h
@@ -36,8 +36,8 @@
 
 __BEGIN_DECLS
 
-int openpty(int*, int*, char*, const struct termios*, const struct winsize*);
-int forkpty(int*, char*, const struct termios*, const struct winsize*);
+int openpty(int*, int*, char*, const struct termios*, const struct winsize*) __INTRODUCED_IN(23);
+int forkpty(int*, char*, const struct termios*, const struct winsize*) __INTRODUCED_IN(23);
 
 __END_DECLS