set use_errno to True for libbcc.so
set use_errno to True for libbcc.so

Currently, errno could not be accessed by libbcc.py, like:
```
binding socket to 'eth0'
cannot create raw socket: Operation not permitted
Traceback (most recent call last):
  File "/home/ryan/Software/bcc/examples/networking/http_filter/http-parse-simple.py", line 73, in <module>
    BPF.attach_raw_socket(function_http_filter, interface)
  File "/usr/lib/python2.7/site-packages/bcc/__init__.py", line 330, in attach_raw_socket
    raise Exception("Failed to open raw device %s: %s" % (dev, errstr))
Exception: Failed to open raw device eth0: Success
```
The exception prints the errno as `Success`, instead of `EPERM`.

This patch would fix this problem.

Signed-off-by: Jincheng Miao <7b92fad43fe66215ca8ce4e2e3d2ef786d7a726d@gmail.com>

brendangregg/bcc,mkacik/bcc,romain-intel/bcc,brendangregg/bcc,mcaleavya/bcc,romain-intel/bcc,shodoco/bcc,tuxology/bcc,mcaleavya/bcc,mcaleavya/bcc,romain-intel/bcc,tuxology/bcc,tuxology/bcc,iovisor/bcc,brendangregg/bcc,mcaleavya/bcc,iovisor/bcc,brendangregg/bcc,mkacik/bcc,mkacik/bcc,romain-intel/bcc,iovisor/bcc,shodoco/bcc,iovisor/bcc,shodoco/bcc,shodoco/bcc,shodoco/bcc,tuxology/bcc,mkacik/bcc,mcaleavya/bcc,romain-intel/bcc,iovisor/bcc,tuxology/bcc,brendangregg/bcc,mkacik/bcc