flow: Fix MSVC compile errors.

flow: Fix MSVC compile errors.

This fixes some MSVC build errors introduced by commit 74ff3298c
(userspace: Define and use struct eth_addr.)

MSVC doesn't like the change in 'const' between function declaration and
definition: it reports "formal parameter 2 different from declaration" for
each of the functions in flow.h corrected by this (commit.  I think it's
technically wrong about that, standards-wise.)

MSVC doesn't like an empty-brace initializer.  (I think it's technically
right about that, standards-wise.)

This commit attempts to fix both problems, but I have not tested it with
MSVC.

CC: Jarno Rajahalme <jrajahalme@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Nithin Raju <nithin@vmware.com>
Tested-by: Nithin Raju <nithin@vmware.com>