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/[e-mail]/
Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Kuan-Wei Chiu <[e-mail]>
Suggested-by: Waiman Long <[e-mail]>
Acked-by: Waiman Long <[e-mail]>
Acked-by: Tejun Heo <[e-mail]>
Reviewed-by: Christoph Hellwig <[e-mail]>
Cc: Ching-Chun (Jim) Huang <[e-mail]>
Cc: Johannes Weiner <[e-mail]>
Cc: Michal Koutný <[e-mail]>
Cc: Xavier <[e-mail]>
Cc: Zefan Li <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

init/Kconfig

···
 config CPUSETS
 	bool "Cpuset controller"
 	depends on SMP
+	select UNION_FIND
 	help
 	  This option will let you create and manage CPUSETs which
 	  allow dynamically partitioning a system into sets of CPUs and

lib/Kconfig

···
 
 config FIRMWARE_TABLE
 	bool
+
+config UNION_FIND
+	bool

lib/Makefile

···
 	 is_single_threaded.o plist.o decompress.o kobject_uevent.o \
 	 earlycpio.o seq_buf.o siphash.o dec_and_lock.o \
 	 nmi_backtrace.o win_minmax.o memcat_p.o \
-	 buildid.o objpool.o union_find.o
+	 buildid.o objpool.o
 
+lib-$(CONFIG_UNION_FIND) += union_find.o
 lib-$(CONFIG_PRINTK) += dump_stack.o
 lib-$(CONFIG_SMP) += cpumask.o