Fix: Change libc error code for process_vm_readv

[omarabid]

At Pyroscope (https://github.com/pyroscope-io/pyroscope), we've been using rbspy with the Rust integration (https://github.com/pyroscope-io/pyroscope-rs). This particular error catch is used to handle a case where SYS_PTRACE is not enabled for docker; and fallback to reading /proc/$pid/mem. From our experience, the relevant error code was libc::EPERM and not libc::ENOSYS.