repart: Remove unnecessary newlines from JSON output

repart: Remove unnecessary newlines from JSON output

Before

```
➜  systemd git:(dissect-have-blkid) SYSTEMD_LOG_LEVEL=err build/systemd-repart --definitions definitions/ --dry-run=yes --empty=create --size=100M --json=pretty abc
[
        {
                "type" : "root-x86-64",
                "label" : "root-x86-64",
                "uuid" : "2f7c902c-6b2a-4b47-a44b-dca765f38cd2",
                "file" : "root.conf",
                "node" : "abc1",
                "offset" : 1048576,
                "old_size" : 0,
                "raw_size" : 103788544,
                "old_padding" : 0,
                "raw_padding" : 0,
                "activity" : "create"
        }
]

➜  systemd git:(dissect-have-blkid)
```

After:

```
➜  systemd git:(repart-newlines) SYSTEMD_LOG_LEVEL=err build/systemd-repart --definitions definitions/ --dry-run=yes --empty=create --size=100M --json=pretty abc
[
        {
                "type" : "root-x86-64",
                "label" : "root-x86-64",
                "uuid" : "2f7c902c-6b2a-4b47-a44b-dca765f38cd2",
                "file" : "root.conf",
                "node" : "abc1",
                "offset" : 1048576,
                "old_size" : 0,
                "raw_size" : 103788544,
                "old_padding" : 0,
                "raw_padding" : 0,
                "activity" : "create"
        }
]
➜  systemd git:(repart-newlines)
```