Add mbtowc and fix mbrtowc.

Change-Id: I48786cd82587e61188d40f6fd6e11ac05e857ae9
diff --git a/libc/Android.mk b/libc/Android.mk
index 90b0d4e..864287a 100644
--- a/libc/Android.mk
+++ b/libc/Android.mk
@@ -336,6 +336,7 @@
     upstream-openbsd/lib/libc/gen/time.c \
     upstream-openbsd/lib/libc/gen/tolower_.c \
     upstream-openbsd/lib/libc/gen/toupper_.c \
+    upstream-openbsd/lib/libc/locale/mbtowc.c \
     upstream-openbsd/lib/libc/locale/wcscoll.c \
     upstream-openbsd/lib/libc/locale/wcsxfrm.c \
     upstream-openbsd/lib/libc/stdio/asprintf.c \