Add Meteosat-12 to platforms.txt

[gerritholl]

• [x] Closes #140
• [ ] Tests added
• [ ] Tests passed
• [x] Passes flake8 pyorbital

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (e76264b) 87.51% compared to head (15e6bb6) 87.51%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #141 +/- ## ======================================= Coverage 87.51% 87.51% ======================================= Files 14 14 Lines 2210 2210 ======================================= Hits 1934 1934 Misses 276 276 ``` | [Flag](https://app.codecov.io/gh/pytroll/pyorbital/pull/141/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytroll) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/pytroll/pyorbital/pull/141/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytroll) | `87.51% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytroll#carryforward-flags-in-the-pull-request-comment) to find out more.

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[coveralls]

coverage: 87.562%. remained the same when pulling 15e6bb65c507bf2763156417f3cac3b99c0a4d10 on gerritholl:meteosat12 into e76264b5d972a620d1abf7ef3e1627eb628b1120 on pytroll:main.

[gerritholl]

Does this need a unit test?

[djhoese]

I vote for no tests needed unless there are already some equivalent/similar tests. I can merge this if you're OK with that @gerritholl.