Fix widgets sample build when using precompiled headers

wx/artprov.h is not included from wx/wx.h, so it needs to be always
included from this header and not only when !WX_PRECOMP.

samples/widgets/headerctrl.cpp

@@ -27,7 +27,6 @@
 
 // for all others, include the necessary headers
 #ifndef WX_PRECOMP
-    #include "wx/artprov.h"
     #include "wx/button.h"
     #include "wx/checkbox.h"
     #include "wx/radiobox.h"
@@ -38,6 +37,8 @@
 
 #include "wx/headerctrl.h"
 
+#include "wx/artprov.h"
+
 #include "widgets.h"
 
 #include "icons/header.xpm"