libseccomp: Update to version 2.4.3

libseccomp: Update to version 2.4.3

- Add list of authorized release signatures to README.md
- Fix multiplexing issue with s390/s390x shm* syscalls
- Remove the static flag from libseccomp tools compilation
- Add define for __SNR_ppoll
- Update our Travis CI configuration to use Ubuntu 18.04
- Disable live python tests in Travis CI
- Use default python, rather than nightly python, in TravisCI
- Fix potential memory leak identified by clang in the scmp_bpf_sim too

The changelog can be found in here https://github.com/seccomp/libseccomp/blob/master/CHANGELOG .

Signed-off-by: Erik Kapfer <ummeegge@ipfire.org>
Reviewed-by: Michael Tremer <michael.tremer@ipfire.org>
Signed-off-by: Arne Fitzenreiter <arne_f@ipfire.org>