kunit: Fix 'hooks.o' build by recursing into kunit

KUnit's 'hooks.o' file need to be built-in whenever KUnit is enabled
(even if CONFIG_KUNIT=m).  We'd previously attemtped to do this by
adding 'kunit/hooks.o' to obj-y in lib/Makefile, but this caused hooks.c
to be rebuilt even when it was unchanged.

Instead, always recurse into lib/kunit using obj-y when KUnit is
enabled, and add the hooks there.

Fixes: 7170b7ed ("kunit: Add "hooks" to call into KUnit when it's built as a module").
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/linux-kselftest/CAHk-=wiEf7irTKwPJ0jTMOF3CS-13UXmF6Fns3wuWpOZ_wGyZQ@mail.gmail.com/


Signed-off-by: David Gow <davidgow@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>