net: xdp: don't export dev_change_xdp_fd()

Since dev_change_xdp_fd() is only used in rtnetlink, which must
be built-in, there's no reason to export dev_change_xdp_fd().

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>