compilation fix of mbiter.h

[Bruno Haible]

mbiter.h and mbuiter.h use ptrdiff_t; this gives a syntax error if <stddef.h>
is not previously included.

2007-06-10  Bruno Haible  <address@hidden>

        * lib/mbiter.h: Include <stddef.h>, needed for ptrdiff_t.
        * lib/mbuiter.h: Likewise.

*** lib/mbiter.h        11 Feb 2007 17:17:09 -0000      1.3
--- lib/mbiter.h        11 Jun 2007 00:15:05 -0000
***************
*** 85,90 ****
--- 85,91 ----
  
  #include <assert.h>
  #include <stdbool.h>
+ #include <stddef.h>
  #include <string.h>
  
  /* Tru64 with Desktop Toolkit C has a bug: <stdio.h> must be included before
*** lib/mbuiter.h       11 Feb 2007 17:17:09 -0000      1.2
--- lib/mbuiter.h       11 Jun 2007 00:15:05 -0000
***************
*** 92,97 ****
--- 92,98 ----
  
  #include <assert.h>
  #include <stdbool.h>
+ #include <stddef.h>
  #include <stdlib.h>
  #include <string.h>