mapi32: Exclude unused header

[Andrew Talbot]

Changelog:
    mapi32: Exclude unused header.

diff -urN a/dlls/mapi32/sendmail.c b/dlls/mapi32/sendmail.c
--- a/dlls/mapi32/sendmail.c	2006-10-10 12:51:45.000000000 +0100
+++ b/dlls/mapi32/sendmail.c	2007-05-08 22:53:50.000000000 +0100
@@ -27,7 +27,6 @@
 #include "windef.h"
 #include "winbase.h"
 #include "winerror.h"
-#include "winnls.h"
 #include "objbase.h"
 #include "mapi.h"
 #include "winreg.h"