Make the bpf filter string for pfring_set_bpf_filter() unmutable.

[utoni]

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• I have updated the documentation (in doc/) to reflect the changes made (if applicable)

Describe changes:

It's better to make the BPF string in pfring_set_bpf_filter() unmutable. It provides a potential developer an easy hint that it's contents won't change while calling that function.
During my PF_RING journey, I've found similar function's/callback's/typedef's that do not need mutable string's/buffer's.