include: Expose <openvswitch/vconn.h> and <openvswitch/ofpbuf.h>

Exposes <openvswitch/vconn.h> and <openvswitch/ofpbuf.h> and drags
along the dependencies <openvswitch/util.h> and <openvswitch/compiler.h>

Signed-off-by: Thomas Graf <tgraf@noironetworks.com>