tools, bpftool: Display license GPL compatible in prog show/list

Display the license "gpl" string in bpftool prog command, like:

  # bpftool prog list
  5: tracepoint  name func  tag 57cd311f2e27366b  gpl
          loaded_at Apr 26/09:37  uid 0
          xlated 16B  not jited  memlock 4096B

  # bpftool --json --pretty prog show
  [{
          "id": 5,
          "type": "tracepoint",
          "name": "func",
          "tag": "57cd311f2e27366b",
          "gpl_compatible": true,
          "loaded_at": "Apr 26/09:37",
          "uid": 0,
          "bytes_xlated": 16,
          "jited": false,
          "bytes_memlock": 4096
      }
  ]

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>