selftests/bpf: add sign-file to .gitignore

Add sign-file to .gitignore to avoid accidentally checking it in.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20220923175913.3272430-2-andrii@kernel.org


Signed-off-by: Alexei Starovoitov <ast@kernel.org>