[BUG] pcn-iptables build instruction is incorrect

[acloudiator]

Describe the bug

Build procedure for pcn-iptables may need a review.

https://github.com/polycube-network/polycube/tree/master/src/components/iptables#steps-to-install-pcn-iptables

To Reproduce

Steps to reproduce the behavior:

1. Follow the instruction from the above link and it won't work, as the build directory isn't there.

Expected behavior

1. Needs a clear and easy to follow instruction to build and install.

Please tell us about your environment:

1. OS details: Ubuntu 18.04 LTS
2. Kernel details: 4.15.0-20-generic x86_64
3. Polycube Version: N/A
4. Polycube Log: N/A

[mauriciovasquezbernal]

Solved by #139