Compile error when NODEMANAGER_SERIAL_INPUT is ON

```
In file included from /github/workspace/examples/Features_ON/Features_ON.ino:176:0:
/github/home/Arduino/libraries/workspace/MySensors_NodeManager.h:68:10: fatal error: core/MyProtocolMySensors.cpp: No such file or directory
 #include "core/MyProtocolMySensors.cpp"
```
