Mark LIBC_N as being introduced in android-24.

Change-Id: I4b8e0ea1fc9b42c1f98f36ae7aaaae2c6322e16b
diff --git a/libc/libc.map.txt b/libc/libc.map.txt
index 261fbe0..2f613b0 100644
--- a/libc/libc.map.txt
+++ b/libc/libc.map.txt
@@ -1226,7 +1226,7 @@
     *;
 };
 
-LIBC_N {
+LIBC_N { # introduced=24
   global:
     __aeabi_atexit; # arm
     __aeabi_memclr; # arm