tap: make struct tap_fops static

The structure tap_fops is local to the source and does not need to
be in global scope, so make it static.

Cleans up sparse warning:
symbol 'tap_fops' was not declared. Should it be static?

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>