wxOS2 with Open Watcom: correct PCH usage, missing headers, warning fixes, source cleaning and other Watcom adjustements.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@34901 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/os2/dnd.cpp

@@ -13,6 +13,11 @@
 #pragma implementation "dnd.h"
 #endif
 
+// For compilers that support precompilation, includes "wx.h".
+#include "wx/wxprec.h"
+
+#if wxUSE_DRAG_AND_DROP
+
 #include "wx/window.h"
 #include "wx/app.h"
 #include "wx/gdicmn.h"
@@ -22,8 +27,6 @@
 #define INCL_DOS
 #include <os2.h>
 
-#if wxUSE_DRAG_AND_DROP
-
 // ----------------------------------------------------------------------------
 // global
 // ----------------------------------------------------------------------------