tun-metadata: Avoid MSVC compile error on 64-bit builds for 0-length array.

tun-metadata: Avoid MSVC compile error on 64-bit builds for 0-length array.

MSVC does not support zero-size arrays except as the last member of a
defined structure.

The error is hit only on MSVC 64 bit because the size of uint64_t is equal
with sizeof(struct tun_table *).

Reported-by: Alin Gabriel Serdean <aserdean@cloudbasesolutions.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Jesse Gross <jesse@nicira.com>