Make multibyte result constants public.

The magic numbers that C defines are obnoxious. We had partial
definitions for these internally. Add the missing one and move them to
a public header for anyone else that may want to use them.

Bug: None
Test: None
Change-Id: Ia6b8cff4310bcccb23078c52216528db668ac966
diff --git a/libc/include/wchar.h b/libc/include/wchar.h
index 2671580..c4e9679 100644
--- a/libc/include/wchar.h
+++ b/libc/include/wchar.h
@@ -37,6 +37,7 @@
 #include <time.h>
 #include <xlocale.h>
 
+#include <bits/bionic_multibyte_result.h>
 #include <bits/mbstate_t.h>
 #include <bits/wchar_limits.h>
 #include <bits/wctype.h>