docs/bpf: Fix sphinx warnings in BPF map docs

Fix duplicate C declaration warnings when using sphinx >= 3.1.

Reported-by: Akira Yokosawa <akiyks@gmail.com>
Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Akira Yokosawa <akiyks@gmail.com>
Link: https://lore.kernel.org/bpf/ed4dac84-1b12-5c58-e4de-93ab9ac67c09@gmail.com
Link: https://lore.kernel.org/bpf/20221122143933.91321-1-donald.hunter@gmail.com