missing #include <unistd.h> in udp.hh

[veto-gh]

Hi there, great little app.
I had issues while compiling on linux.
Adding #include <unistd.h> to udp.hh solved that.
Cheers

[icq4ever]

@veto-gh thanks!!

[mhill900]

This project seems abandoned.
Fixed it for me anyways and made a PR (#3).