tun-device: Include system headers before our own

tun-device: Include system headers before our own

On CentOS 6.5 the sys/capability.h header file defines _LINUX_TYPES_H
without actually including that header, preventing its later inclusion
here.

As library.h (via which the capabilities headers are included) is not
actually required in tun_device.[ch], moving the inclusion of tun_device.h
would not strictly be necessary.  But it's probably a good idea to
include our own headers after system headers anyway, for if one of the
recursively included files at a later point includes library.h we'd have
the same problem again.