packets: Fix C++ compilation issues when include packets.h

packets: Fix C++ compilation issues when include packets.h

This patch fixes three C++ compilation errors when it includes
"lib/packets.h".

1) Fix in "include/openvswitch/util.h" is to avoid duplicated
named_member__ in struct pkt_metadata.

2) Fix in "lib/packets.h" is because designated initializers are not
implemented in GNU C++ [1].

3) Fix in "lib/util.h" is because __builtin_types_compatible_p and
__builtin_choose_expr are only supported in GCC. I use one solution
for C++ that is type-safe and works at compile time from [2].

[1]: https://gcc.gnu.org/onlinedocs/gcc/Designated-Inits.html
[2]: https://goo.gl/xNe48A

Signed-off-by: Yi-Hung Wei <yihung.wei@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>