lib/Makefile: make union-find compilation conditional on CONFIG_CPUSETS

Currently, cpuset is the only user of the union-find implementation. 
Compiling union-find in all configurations unnecessarily increases the
code size when building the kernel without cgroup support.  Modify the
build system to compile union-find only when CONFIG_CPUSETS is enabled.

Link: https://lore.kernel.org/lkml/1ccd6411-5002-4574-bb8e-3e64bba6a757@redhat.com/
Link: https://lkml.kernel.org/r/20241011141214.87096-1-visitorckw@gmail.com


Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
Suggested-by: Waiman Long <llong@redhat.com>
Acked-by: Waiman Long <longman@redhat.com>
Acked-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: Ching-Chun (Jim) Huang <jserv@ccns.ncku.edu.tw>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Koutný <mkoutny@suse.com>
Cc: Xavier <xavier_qy@163.com>
Cc: Zefan Li <lizefan.x@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>