Add POSIX getsubopt(3).

Bug: http://b/27952303
Change-Id: I8a816477545dadcbd5c055714e76263574446b6f
diff --git a/libc/libc.x86.brillo.map b/libc/libc.x86.brillo.map
index c67f533..9c0b2ec 100644
--- a/libc/libc.x86.brillo.map
+++ b/libc/libc.x86.brillo.map
@@ -1257,6 +1257,7 @@
 LIBC_O {
   global:
     getdomainname;
+    getsubopt;
     hasmntopt;
     pthread_getname_np;
     setdomainname;