tools: ynl-gen: add missing request free helpers for dumps

The code gen generates a prototype for dump request free
in the header, but no implementation in the source.

Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://lore.kernel.org/r/20231213231432.2944749-2-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>