bpf: annotate bpf_insn_print_t with __printf

Functions of type bpf_insn_print_t take printf-like format
string, mark the type accordingly.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>