tools: ynl: fix build of the page-pool sample

tools: ynl: fix build of the page-pool sample

The name of the "destroyed" field in the reply was not changed
in the sample after we started calling it "detach_time".

page-pool.c: In function ‘main’:
page-pool.c:84:33: error: ‘struct <anonymous>’ has no member named ‘destroyed’
   84 |                 if (pp->_present.destroyed)
      |                                 ^

Fixes: 637567e4a3ef ("tools: ynl: add sample for getting page-pool information")
Link: https://lore.kernel.org/r/20231129193622.2912353-2-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>