Changes

core: always build tun.c and gate TUN support by headers

Always build tun.c and move platform-specific checks into the
implementation.

Guard the TUN-specific code paths based on the availability of
linux/if_tun.h detected at configure time. If TUN support is not
available, osmo_tundev_open() returns -ENOTSUP.

This keeps the public TUN API available while disabling unsupported
functionality in a capability-based way.

Change-Id: I6d1ea1644d12ef59a54cf2f73b9155def58b17a9