New license request: gpl-2.0-plus variant

**License Name**

A variant for GPL-2.0-plus found in squashfs-tools at:

https://github.com/plougher/squashfs-tools/blob/master/squashfs-tools/xz_wrapper_extended.c#L5

Scancode currently gives a partial match to:

https://github.com/aboutcode-org/scancode-toolkit/blob/develop/src/licensedcode/data/rules/gpl-2.0-plus_239.RULE

```
"license_detections": [
{
"license_expression": "gpl-2.0-plus",
"license_expression_spdx": "GPL-2.0-or-later",
"matches": [
{
"license_expression": "gpl-2.0-plus",
"license_expression_spdx": "GPL-2.0-or-later",
"from_file": "squashfs-tools/squashfs-tools/xz_wrapper_extended.c",
"start_line": 5,
"end_line": 17,
"matcher": "3-seq",
"score": 96.46,
"matched_length": 109,
"match_coverage": 96.46,
"rule_relevance": 100,
"rule_identifier": "gpl-2.0-plus_239.RULE",
"rule_url": "https://github.com/nexB/scancode-toolkit/tree/develop/src/licensedcode/data/rules/gpl-2.0-plus_239.RULE"
}
],
"identifier": "gpl_2_0_plus-1cc87c17-6c55-e17e-bfe8-af56fd47cf42"
}
```

This is the wrong rule, because the rule explicitly mentions a referenced file name, while the file I am scanning from squashfs-tools does not reference this file name.

This is fixed by #4947