add function for adding file descriptor to poller_t

[uyha]

Fix #557

[coveralls]

Pull Request Test Coverage Report for Build 5201493977

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 86.251%

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Totals
Change from base Build 5121755673:	0.0%
Covered Lines:	803
Relevant Lines:	931

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💛 - Coveralls

[uyha]

@gummif I don't think the runners are gonna pick this up :sweat_smile:

[uyha]

it seems like Github discontinued ubuntu-18.04, it's not listed in here anymore.

[gummif]

Ok thanks, will fix that later.