s390/purgatory: hard-code obj-y in Makefile

The purgatory/ directory is entirely guarded in arch/s390/Kbuild.
CONFIG_ARCH_HAS_KEXEC_PURGATORY is bool type.

$(CONFIG_ARCH_HAS_KEXEC_PURGATORY) is always 'y' when Kbuild visits
this Makefile for building.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Link: https://lore.kernel.org/r/20220613170902.1775211-2-masahiroy@kernel.orgSigned-off-by: Alexander Gordeev <agordeev@linux.ibm.com>

arch/s390/purgatory/Makefile

@@ -51,4 +51,4 @@ $(obj)/purgatory.ro: $(obj)/purgatory $(obj)/purgatory.chk FORCE
 $(obj)/kexec-purgatory.o: $(obj)/kexec-purgatory.S $(obj)/purgatory.ro FORCE
 	$(call if_changed_rule,as_o_S)
 
-obj-$(CONFIG_ARCH_HAS_KEXEC_PURGATORY) += kexec-purgatory.o
+obj-y += kexec-purgatory.o