Added beacon packet to handshake cap file for PMKID assoc attack

[dandare100]

Hello
Regarding #592

I was also having a problem completing this example

https://www.evilsocket.net/2019/02/13/Pwning-WiFi-networks-with-bettercap-and-the-PMKID-client-less-attack/

where there was no output file created from
hcxpcaptool -k pmkid.16800

It seems the beacon packet was missing from the handshake file.

This merge request adds the packet to the handshake file.

I have tested it locally and the hcxpcaptool now writes the hash to the file once the beacon packet is included

What an awesome project !! Thank you for it.

[dStruct]

I was affected by this, and applied this fix and it absolutely resolved the issue on Ubuntu 18.04 LTS.