nvtop: new port (v3.1.0)

[herbygillot]

https://github.com/Syllo/nvtop

Verification

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• followed our Commit Message Guidelines?
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• checked that there aren't other open pull requests for the same change?
• referenced existing tickets on Trac with full URL in commit message?
• checked your Portfile with port lint --nitpick?
• tried existing tests with sudo port test sudo port -d destroot?
• tried a full install with sudo port -vst install?
• tested basic functionality of all binary files?
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[ryandesign]

CI succeeded on macOS 13 and 14 but failed on macOS 11 because error: use of undeclared identifier 'kIOMainPortDefault'. Indeed according to Apple documentation kIOMainPortDefault was introduced in macOS 12. Could you indicate this with platforms {darwin >= 21}?

[ryandesign]

I filed an upstream bug report: Syllo/nvtop#284

[herbygillot]

platforms {darwin >= 21}

Added, thanks for taking a look