Won't compile for Ubuntu

Hello,
Was getting an error when trying to compile for Ubuntu, saying it could not find QSerialPort

The fix is to open src/ubuntu/serial.h
Change line 28 to read  

```
#include <QtSerialPort/QSerialPort>
```

This was with the latest version of Linux Mint.  I'd submit a PR, but this is such a simple fix I figured it didn't warrant a PR.
