winmm: Some 64-bit pointer fixes.
diff --git a/include/mmddk.h b/include/mmddk.h
index 57d3e4b..c2ed4fc 100644
--- a/include/mmddk.h
+++ b/include/mmddk.h
@@ -413,8 +413,8 @@
 {
 	HMIXEROBJ		hmx;
         LPVOID			pReserved0;
-	DWORD			dwCallback;
-	DWORD			dwInstance;
+	DWORD_PTR		dwCallback;
+	DWORD_PTR		dwInstance;
 } MIXEROPENDESC, *LPMIXEROPENDESC;
 
 typedef struct {