lib/posix-process: Fix `vfork` signature

lib/posix-process: Fix `vfork` signature

A minor oversight during merging of the `vfork` functionality ended
up defining `vfork` with an invalid signature that takes two arguments.

Fix this by defining `vfork` as a syscall that takes no arguments.

Signed-off-by: Sergiu Moga <sergiu@unikraft.io>
Approved-by: Michalis Pappas <michalis@unikraft.io>
Reviewed-by: Michalis Pappas <michalis@unikraft.io>
Reviewed-by: Andrei Tatar <andrei@unikraft.io>
GitHub-Closes: #1618