Add API constant for S.

Test: treehugger
Bug: http://b/152037166
Change-Id: I9039377e80574cd59e13726a12072fe3002df67d
diff --git a/libc/libc.map.txt b/libc/libc.map.txt
index 2284775..10732a1 100644
--- a/libc/libc.map.txt
+++ b/libc/libc.map.txt
@@ -1550,7 +1550,7 @@
     _Unwind_VRS_Set; # apex llndk arm
 } LIBC_Q;
 
-LIBC_S { # introduced=31
+LIBC_S { # introduced=S
   global:
     ffsl;
     ffsll;