compiler: Fix compilation when using VStudio 2015/2017

compiler: Fix compilation when using VStudio 2015/2017

This is somewhat a regression of:
https://github.com/openvswitch/ovs/commit/27f141d44d95b4cabfd7eac47ace8d1201668b2c

The main issue using `offsetof` from <stddef.h> via the C compiler from
MSVC 2015/2017 has issues and is buggy:
https://bit.ly/2UvWwti

Until it is fixed, we define our own definition of `offsetof`.

Signed-off-by: Alin Gabriel Serdean <aserdean@ovn.org>
Acked-by: Ben Pfaff <blp@ovn.org>
Acked-by: Anand Kumar <kumaranand@vmware.com>