packet: Avoid array of struct with zero length member.

packet: Avoid array of struct with zero length member.

Windows doesn't like that the Geneve header has an array of
options with each have a zero length member (the variable data).
Nothing is accessing the data now, so just replace the member with
a comment - we can use pointer arithmetic when necessary.

Reported-by: Gurucharan Shetty <shettyg@nicira.com>
Signed-off-by: Jesse Gross <jesse@nicira.com>