add newly created Linux files

linux/polyvox_filament_plugin_private.h

@@ -0,0 +1,10 @@
+#include <flutter_linux/flutter_linux.h>
+
+#include "include/polyvox_filament/polyvox_filament_plugin.h"
+
+// This file exposes some plugin internals for unit testing. See
+// https://github.com/flutter/flutter/issues/88724 for current limitations
+// in the unit-testable API.
+
+// Handles the getPlatformVersion method call.
+FlMethodResponse *get_platform_version();