Add versioning information to symbols.

Bug: http://b/28178111
Change-Id: I46bf95accd819f4521afb1173d8badcc5e9df31c
diff --git a/libc/include/net/if.h b/libc/include/net/if.h
index aa4c19e..eac7699 100644
--- a/libc/include/net/if.h
+++ b/libc/include/net/if.h
@@ -46,8 +46,8 @@
 
 char* if_indextoname(unsigned, char*);
 unsigned if_nametoindex(const char*);
-struct if_nameindex* if_nameindex(void);
-void if_freenameindex(struct if_nameindex*);
+struct if_nameindex* if_nameindex(void) __INTRODUCED_IN(24);
+void if_freenameindex(struct if_nameindex*) __INTRODUCED_IN(24);
 
 __END_DECLS