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: NChad Williamson <chad@dahc.us>
Reviewed-By: NWill Drewry <wad@chromium.org>
Acked-by: NKees Cook <keescook@chromium.org>
Signed-off-by: NJames Morris <james.l.morris@oracle.com>

samples/seccomp/.gitignore

+bpf-direct
+bpf-fancy
+dropper