Implement mblen(3).

Change-Id: I65948ea5b9ecd63f966ba767ad6db4a2effc4700
diff --git a/libc/Android.mk b/libc/Android.mk
index 374ffa6..ac7cd75 100644
--- a/libc/Android.mk
+++ b/libc/Android.mk
@@ -133,6 +133,7 @@
     bionic/lockf.cpp \
     bionic/lstat.cpp \
     bionic/malloc_info.cpp \
+    bionic/mblen.cpp \
     bionic/mbrtoc16.cpp \
     bionic/mbrtoc32.cpp \
     bionic/mbstate.cpp \