tools: bpftool: fix warning on struct bpf_prog_linfo definition

The following warning appears when compiling bpftool without BFD
support:

main.h:198:23: warning: 'struct bpf_prog_linfo' declared inside
    parameter list will not be visible outside of this definition or
    declaration
          const struct bpf_prog_linfo *prog_linfo,

Fix it by declaring struct bpf_prog_linfo even in the case BFD is not
supported.

Fixes: b053b439

 ("bpf: libbpf: bpftool: Print bpf_line_info during prog dump")
Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>