include/uapi/linux/openvswitch.h: use __u32 from linux/types.h

Kernel uapi header are supposed to use them. Fixes userspace compile error:

linux/openvswitch.h:583:2: error: unknown type name ‘uint32_t’

Signed-off-by: Mikko Rapeli <mikko.rapeli@iki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>