net: Remove unneeded export of a couple of xdp generic functions

generic_xdp_tx and xdp_do_generic_redirect are only used by builtin
code, so remove the EXPORT_SYMBOL_GPL for them.

Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>