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/bionic/mbrtoc32.cpp b/libc/bionic/mbrtoc32.cpp
index d37ca66..c26dd71 100644
--- a/libc/bionic/mbrtoc32.cpp
+++ b/libc/bionic/mbrtoc32.cpp
@@ -109,7 +109,7 @@
     mbstate_set_byte(state, bytes_so_far + i, *s++);
   }
   if (i < bytes_wanted) {
-    return __MB_ERR_INCOMPLETE_SEQUENCE;
+    return BIONIC_MULTIBYTE_RESULT_INCOMPLETE_SEQUENCE;
   }
 
   // Decode the octet sequence representing the character in chunks