selftests: make kselftest-clean remove libynl outputs

Starting with 6.12 commit 85585b4 ("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: 85585b4 ("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>

tools/testing/selftests/net/ynl.mk

@@ -19,3 +19,7 @@ $(YNL_OUTPUTS): CFLAGS += \
 $(OUTPUT)/libynl.a:
 	$(Q)$(MAKE) -C $(top_srcdir)/tools/net/ynl GENS="$(YNL_GENS)" libynl.a
 	$(Q)cp $(top_srcdir)/tools/net/ynl/libynl.a $(OUTPUT)/libynl.a
+
+EXTRA_CLEAN += \
+	$(top_srcdir)/tools/net/ynl/lib/__pycache__ \
+	$(top_srcdir)/tools/net/ynl/lib/*.[ado]