include: Undefine INTERFACE before defining it.

include/ddraw.h

diff --git a/include/ddraw.h b/include/ddraw.h
index eea7e09..88990f0 100644
--- a/include/ddraw.h
+++ b/include/ddraw.h
@@ -1220,6 +1220,7 @@
 /*****************************************************************************
  * IDirectDrawPalette interface
  */
+#undef INTERFACE
 #define INTERFACE IDirectDrawPalette
 DECLARE_INTERFACE_(IDirectDrawPalette,IUnknown)
 {