selftests/bpf: Fix spelling mistake "get_signaure_size" -> "get_signature_size"

There is a spelling mistake in an ASSERT_GT message. Fix it.

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20231204093940.2611954-1-colin.i.king@gmail.com