samples: seccomp: add .gitignore for untracked executables

git status should be clean following make allmodconfig && make. Add
a .gitignore file to the samples/seccomp directory to ignore binaries
produced there.

Signed-off-by: Chad Williamson <chad@dahc.us>
Reviewed-By: Will Drewry <wad@chromium.org>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: James Morris <james.l.morris@oracle.com>