fix -Wenum-compare-switch warnings
fix -Wenum-compare-switch warnings

```
/vagrant/src/attached_probe.cpp:113:10: warning: comparison of different enumeration types in switch statement ('bpf_prog_type' and 'libbpf::bpf_prog_type') [-Wenum-compare-switch]
    case libbpf::BPF_PROG_TYPE_TRACING:    return "BPF_PROG_TYPE_TRACING";    break;
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/vagrant/src/attached_probe.cpp:112:10: warning: comparison of different enumeration types in switch statement ('bpf_prog_type' and 'libbpf::bpf_prog_type') [-Wenum-compare-switch]
    case libbpf::BPF_PROG_TYPE_PERF_EVENT: return "BPF_PROG_TYPE_PERF_EVENT"; break;
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/vagrant/src/attached_probe.cpp:111:10: warning: comparison of different enumeration types in switch statement ('bpf_prog_type' and 'libbpf::bpf_prog_type') [-Wenum-compare-switch]
    case libbpf::BPF_PROG_TYPE_TRACEPOINT: return "BPF_PROG_TYPE_TRACEPOINT"; break;
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/vagrant/src/attached_probe.cpp:110:10: warning: comparison of different enumeration types in switch statement ('bpf_prog_type' and 'libbpf::bpf_prog_type') [-Wenum-compare-switch]
    case libbpf::BPF_PROG_TYPE_KPROBE:     return "BPF_PROG_TYPE_KPROBE";     break;
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
4 warnings generated.
```

iovisor/bpftrace,iovisor/bpftrace,iovisor/bpftrace,iovisor/bpftrace