check-structs: Disallow uint<N>_t because ovs_be<N> should always be used.

The header files that check-structs checks should only contain big-endian
data, never native-endian data, so disallow uint<N>_t entirely.  (We had
a couple of mistakes in this area until recently.)

uint8_t is an obvious exception.

Reported-by: Simon Horman <horms@verge.net.au>
Reviewed-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Ben Pfaff <blp@nicira.com>