WebView: Only include the private header on Windows and macOS

The wrapper isn't used in the Linux code path. Some distributions'
wxWidgets packages don't ship the private headers.

src/slic3r/GUI/Widgets/WebView.cpp

@@ -8,7 +8,9 @@
 #include <wx/msw/webview_edge.h>
 #endif
 #include <wx/uri.h>
+#if defined(__WIN32__) || defined(__WXMAC__)
 #include "wx/private/jsscriptwrapper.h"
+#endif
 
 #ifdef __WIN32__
 #include "../WebView2.h"