[stubs] include unistd.h to repair swift-threading-package=none build

[kateinoigakukun]

syscall and read were missing when swift-threading-package=none since #59287 has been merged.
It results build failure with such configuration: e.g. https://ci-external.swift.org/job/oss-swift-RA-linux-ubuntu-20.04-webassembly/407/console

/home/buildbot/jenkins/workspace/oss-swift-RA-linux-ubuntu-20.04-webassembly/swift/stdlib/public/stubs/Random.cpp:93:9: error: use of undeclared identifier 'syscall'
      !(syscall(__NR_getrandom, nullptr, 0, 0) == -1 && errno == ENOSYS);
        ^
/home/buildbot/jenkins/workspace/oss-swift-RA-linux-ubuntu-20.04-webassembly/swift/stdlib/public/stubs/Random.cpp:96:35: error: use of undeclared identifier 'syscall'
      actual_nbytes = WHILE_EINTR(syscall(__NR_getrandom, buf, nbytes, 0));
                                  ^
/home/buildbot/jenkins/workspace/oss-swift-RA-linux-ubuntu-20.04-webassembly/swift/stdlib/public/stubs/Random.cpp:96:35: error: use of undeclared identifier 'syscall'
/home/buildbot/jenkins/workspace/oss-swift-RA-linux-ubuntu-20.04-webassembly/swift/stdlib/public/stubs/Random.cpp:96:35: error: use of undeclared identifier 'syscall'
/home/buildbot/jenkins/workspace/oss-swift-RA-linux-ubuntu-20.04-webassembly/swift/stdlib/public/stubs/Random.cpp:114:39: error: use of undeclared identifier 'read'; did you mean '_IO_cookie_io_functions_t::read'?
          actual_nbytes = WHILE_EINTR(read(fd, buf, nbytes));
                                      ^~~~
                                      _IO_cookie_io_functions_t::read

[kateinoigakukun]

@swift-ci Please smoke test

[kateinoigakukun]

preset=buildbot_incremental_linux_crosscompile_wasm
@swift-ci Please test with preset Linux Platform

[3405691582]

FTR, this is also required for swift-threading-package=pthreads.

[al45tair]

I think maybe the threading unit tests need a tweak in this case too. There's probably not much point in running them in the swift-threading-package=none case.

[kateinoigakukun]

Yeah, I'll tweak the test later (I don't have enough time right now sorry)

[al45tair]

I think this is a pretty safe change anyway, even without fixing the tests. I'll probably take a look at the test problem tomorrow.

[kateinoigakukun]

Thank you @al45tair !

[al45tair]

Hopefully #59403 will fix the test problem.