Add the Gre header structure in librte_net. It will be used by next
patches that adds the support of Gre tunnels in the software packet type
parser.
The extended headers (checksum, key or sequence number) are not defined.
Signed-off-by: Jean Dao <jean.dao@6wind.com>
Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
