Fix missing imports in posix environment

[fox0430]

Fix a problem that can't compile with the following error.

$ nim -v

Nim Compiler Version 1.6.12 [Linux: amd64]
Compiled at 2023-03-10
Copyright (c) 2006-2023 by Andreas Rumpf

git hash: 1aa9273640c0c51486cf3a7b67282fe58f360e91
active boot switches: -d:release

$ cd my-app
$ nimble build
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.
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/home/fox/.nimble/pkgs/chronos-3.1.0/chronos/selectors2.nim(298, 12) Error: undeclared identifier: 'sigprocmask'
candidates (edit distance, scope distance); see '--spellSuggest':
 (6, 6): 'EINPROGRESS' [const declared in /home/fox/.nimble/pkgs/chronos-3.1.0/chronos/oserrno.nim(1274, 5)]
 (6, 6): 'EINPROGRESS' [const declared in /home/fox/.nimble/pkgs/chronos-3.1.0/chronos/oserrno.nim(1274, 5)]
 (6, 6): 'EINPROGRESS' [const declared in /home/fox/.nimble/pkgs/chronos-3.1.0/chronos/oserrno.nim(1274, 5)]
 (6, 6): 'SIG_BLOCK' [const declared in /home/fox/.choosenim/toolchains/nim-1.6.12/lib/posix/posix_linux_amd64_consts.nim(403, 7)]
 (6, 6): 'SIG_BLOCK' [const declared in /home/fox/.choosenim/toolchains/nim-1.6.12/lib/posix/posix_linux_amd64_consts.nim(403, 7)]
 (6, 6): 'procCall' [proc declared in /home/fox/.choosenim/toolchains/nim-1.6.12/lib/system.nim(2965, 6)]
       Tip: 750 messages have been suppressed, use --verbose to show them.
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[arnetheduck]

396 fixes this in a different way