Implement mblen(3).

Change-Id: I65948ea5b9ecd63f966ba767ad6db4a2effc4700
diff --git a/libc/libc.map.txt b/libc/libc.map.txt
index be37ffb..8d1a1f9 100644
--- a/libc/libc.map.txt
+++ b/libc/libc.map.txt
@@ -1304,6 +1304,7 @@
     getpwent;
     getsubopt;
     hasmntopt;
+    mblen;
     pthread_getname_np;
     quotactl;
     setdomainname;