libbcc-py: Fix libbpf types

libbcc-py: Fix libbpf types

1ad2656a1d9c ("Add support_kfunc function to BPF object") added new functions
to libbcc-py but didn't set the restype and argstypes for those. It's causing
the `bpf_has_kernel_btf` function to return True in systems without support
for BTF, making tools like opensnoop, klockstat and any other using kfuncs
unusable in those systems.

The following Python script reproduces the problem:

```
from bcc import BPF
print(BPF.support_kfunc())
```

Signed-off-by: Mauricio Vásquez <mauricio@kinvolk.io>