Add values to describe return values from WM_NOTIFY of TBN_DROPDOWN.

diff --git a/include/commctrl.h b/include/commctrl.h
index a33e365..fa8a854 100644
--- a/include/commctrl.h
+++ b/include/commctrl.h
@@ -1036,6 +1036,11 @@
 #define TBN_GETINFOTIP WINELIB_NAME_AW(TBN_GETINFOTIP)
 
 
+/* Return values from TBN_DROPDOWN */
+#define TBDDRET_DEFAULT  0
+#define TBDDRET_NODEFAULT  1
+#define TBDDRET_TREATPRESSED  2
+
 typedef struct _NMTBCUSTOMDRAW
 {
     NMCUSTOMDRAW nmcd;