packets: Do not use zero sized array in icmp header.

packets: Do not use zero sized array in icmp header.

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

icmp headers got included inside 'struct ovs_nd_msg' through
commit e60e935b1f (Implement set-field for IPv6 ND fields (nd_target,
nd_sll,and nd_tll). This causes compilation error while using MSVC.

Since icmp[6]_data in the icmp[6]_header is not used anywhere, just remove
them.

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