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

`syscall` and `read` were missing when `swift-threading-package=none`

glessard/swift,glessard/swift,roambotics/swift,apple/swift,apple/swift,apple/swift,benlangmuir/swift,JGiola/swift,glessard/swift,benlangmuir/swift,roambotics/swift,benlangmuir/swift,JGiola/swift,benlangmuir/swift,roambotics/swift,apple/swift,JGiola/swift,benlangmuir/swift,glessard/swift,benlangmuir/swift,JGiola/swift,roambotics/swift,apple/swift,apple/swift,roambotics/swift,glessard/swift,glessard/swift,roambotics/swift,JGiola/swift,JGiola/swift