Don't define BEGIN_INTERFACE in unknwn.h.

diff --git a/dlls/dmscript/dmscript_private.h b/dlls/dmscript/dmscript_private.h
index 51f1c1c..b24020c 100644
--- a/dlls/dmscript/dmscript_private.h
+++ b/dlls/dmscript/dmscript_private.h
@@ -36,6 +36,7 @@
 #include "wine/list.h"
 #include "wine/unicode.h"
 #include "winreg.h"
+#include "objbase.h"
 
 #include "dmusici.h"
 #include "dmusicf.h"