Seccomp dependency not listed in README

[Calle Svensson]

Trying to compile on a fresh system results in:

deexec.c:9:10: fatal error: seccomp.h: No such file or directory
 #include <seccomp.h>
          ^~~~~~~~~~~
compilation terminated.

Which is fixed by (on debian based systems):
sudo apt install libseccomp-dev

This should be added to the README. I'll do a PR later if no one has fixed it before I get around to it.