Missing psutil dependency in requirements.txt

## Description

`ultralytics/yolo/utils/checks.py` imports `psutil`, but it is not listed in `requirements.txt`. This causes a `ModuleNotFoundError` on fresh installs.

## Error

```
ModuleNotFoundError: No module named 'psutil'
```

## Location

`ultralytics/yolo/utils/checks.py` line 17:
```python
import psutil
```

## Suggested Fix

Add `psutil` to `requirements.txt`.
