ynl: include uapi header after all dependencies

Essentially reverse the order of headers for userspace generated files.

Before (make -C tools/net/ynl/; cat tools/net/ynl/ethtool-user.h):
  #include <linux/ethtool_netlink_generated.h>
  #include <linux/ethtool.h>
  #include <linux/ethtool.h>
  #include <linux/ethtool.h>

After:
  #include <linux/ethtool.h>
  #include <linux/ethtool_netlink_generated.h>

While at it, make sure we track which headers we've already included
and include the headers only once.

Signed-off-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20241204155549.641348-6-sdf@fomichev.me


Signed-off-by: Jakub Kicinski <kuba@kernel.org>