tools: ynl: make sure we use local headers for page-pool

tools: ynl: make sure we use local headers for page-pool

Building samples generates the following warning:

  In file included from page-pool.c:11:
  generated/netdev-user.h:21:45: warning: ‘enum netdev_xdp_rx_metadata’ declared inside parameter list will not be visible outside of this definition or declaration
   21 | const char *netdev_xdp_rx_metadata_str(enum netdev_xdp_rx_metadata value);
      |                                             ^~~~~~~~~~~~~~~~~~~~~~

Our magic way of including uAPI headers assumes the sample
name matches the family name. We need to copy the flags over.

Fixes: 637567e4a3ef ("tools: ynl: add sample for getting page-pool information")
Link: https://lore.kernel.org/r/20231129193622.2912353-3-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>