selftests: net: rebuild YNL if dependencies changed

selftests: net: rebuild YNL if dependencies changed

Try to rebuild YNL if either user added a new family or the specs
of the families have changed. Stanislav's ncdevmem cause a false
positive build failure in NIPA because libynl.a isn't rebuilt
after ethtool is added to YNL_GENS.

Note that sha1sum is already used in other parts of the build system.

Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20241011230311.2529760-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>