ofp-util: compile group stats with visual studio

Visual studio supports zero-size array within a struct or union,
but has to be the last element. GCC does not have this restriction.

Commits 644cfd84772eb7d8 and 6fdaa45a6f6c9 make use of 0 size array.
Remove them so that visual studio can compile them as well.

Reported-by: Gurucharan Shetty <gshetty@nicira.com>
Signed-off-by: Andy Zhou <azhou@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>