Fix wxGTK build after wxLaunchDefaultBrowser() fix backport.

2dd407609b uses wxGetTopLevelGDK() only
available in master and not in 3.0 branch, avoid it by just using the default
screen (which also avoids having to add another GTK+ version check for
gdk_window_get_screen() which is only available since 2.24).

src/unix/utilsx11.cpp

@@ -894,8 +894,7 @@ bool wxDoLaunchDefaultBrowser(const wxString& url, int flags)
     if (gtk_check_version(2,14,0) == NULL)
 #endif
     {
-        GdkScreen* screen = gdk_window_get_screen(wxGetTopLevelGDK());
-        if (gtk_show_uri(screen, url.utf8_str(), GDK_CURRENT_TIME, NULL))
+        if (gtk_show_uri(NULL, url.utf8_str(), GDK_CURRENT_TIME, NULL))
             return true;
     }
 #endif // GTK_CHECK_VERSION(2,14,0)