Moved HDROP to shellapi.h.

diff --git a/include/shellapi.h b/include/shellapi.h
index b964fc8..03b1b2c 100644
--- a/include/shellapi.h
+++ b/include/shellapi.h
@@ -9,6 +9,8 @@
 
 #include "pshpack1.h"
 
+DECLARE_HANDLE(HDROP);
+
 /******************************************
  * DragObject
  */