Added undocumented window style.

diff --git a/include/winuser.h b/include/winuser.h
index ac8be48..78fecf8 100644
--- a/include/winuser.h
+++ b/include/winuser.h
@@ -276,6 +276,7 @@
 #define ES_AUTOVSCROLL  0x00000040
 #define ES_AUTOHSCROLL  0x00000080
 #define ES_NOHIDESEL    0x00000100
+#define ES_COMBO        0x00000200   /* Undocumented. Parent is a combobox */
 #define ES_OEMCONVERT   0x00000400
 #define ES_READONLY     0x00000800
 #define ES_WANTRETURN   0x00001000