Don't include all wx/xxx/webviewhistoryitem_xxx.h headers.

Only one of these headers is available for each platform so include at most
one of them.

Closes #13705.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69873 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/webview.h

@@ -20,9 +20,15 @@
 #include "wx/sharedptr.h"
 #include "wx/vector.h"
 
+#if defined(__WXOSX__)
     #include "wx/osx/webviewhistoryitem_webkit.h"
+#elif defined(__WXGTK__)
     #include "wx/gtk/webviewhistoryitem_webkit.h"
+#elif defined(__WXMSW__)
     #include "wx/msw/webviewhistoryitem_ie.h"
+#else
+    #error "wxWebView not implemented on this platform."
+#endif
 
 class wxFSFile;
 class wxFileSystem;