selftests: make kselftest-clean remove libynl outputs

selftests: make kselftest-clean remove libynl outputs

Starting with 6.12 commit 85585b4bc8d8 ("selftests: add ncdevmem, netcat
for devmem TCP") kselftest-all creates additional outputs that
kselftest-clean does not cleanup:
  $ make defconfig
  $ make kselftest-all
  $ make kselftest-clean
  $ git clean -ndxf | grep tools/net
  Would remove tools/net/ynl/lib/__pycache__/
  Would remove tools/net/ynl/lib/ynl.a
  Would remove tools/net/ynl/lib/ynl.d
  Would remove tools/net/ynl/lib/ynl.o

Make kselftest-clean remove the newly added net/ynl outputs.

Fixes: 85585b4bc8d8 ("selftests: add ncdevmem, netcat for devmem TCP")
Signed-off-by: Greg Thelen <gthelen@google.com>
Reviewed-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://patch.msgid.link/20241005215600.852260-1-gthelen@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>