kunit: config: KUNIT_DEBUGFS should depend on DEBUG_FS

CONFIG_KUNIT_DEBUGFS is totally useless without debugfs, so it should
depend on CONFIG_DEBUG_FS.

Link: https://lore.kernel.org/r/20260425034155.53913-2-david@davidgow.net
Fixes: e2219db280e3 ("kunit: add debugfs /sys/kernel/debug/kunit/<suite>/results display")
Signed-off-by: David Gow <david@davidgow.net>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>