Seccomp dependency not listed in README

[Tom Tan]

Try make on a fresh system with the dependencies listed in README installed, and got below error. This could be fixed by running sudo apt install libseccomp-dev on debian.

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

[Daniel J Walsh]

Please open a PR to add the dependancy.