Added a missing define.

diff --git a/include/mmddk.h b/include/mmddk.h
index 48f12b0..32e5f25 100644
--- a/include/mmddk.h
+++ b/include/mmddk.h
@@ -93,6 +93,8 @@
 #define DRV_QUERYDSOUNDDESC		(DRV_RESERVED + 21)
 #endif
 
+#define DRVM_MAPPER_PREFERRED_FLAGS_PREFERREDONLY   0x00000001
+
 #define WODM_INIT		DRVM_INIT
 #define WODM_GETNUMDEVS		 3
 #define WODM_GETDEVCAPS		 4