ia64: simplify esi object addition in Makefile

CONFIG_IA64_ESI is a bool option. I do not know why the Makefile was
written like this, but this should not have any functional change.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>

arch/ia64/kernel/Makefile

@@ -34,10 +34,7 @@ mca_recovery-y			+= mca_drv.o mca_drv_asm.o
 obj-$(CONFIG_IA64_MC_ERR_INJECT)+= err_inject.o
 obj-$(CONFIG_STACKTRACE)	+= stacktrace.o
 
-obj-$(CONFIG_IA64_ESI)		+= esi.o
-ifneq ($(CONFIG_IA64_ESI),)
-obj-y				+= esi_stub.o	# must be in kernel proper
-endif
+obj-$(CONFIG_IA64_ESI)		+= esi.o esi_stub.o # must be in kernel proper
 obj-$(CONFIG_INTEL_IOMMU)	+= pci-dma.o
 
 obj-$(CONFIG_ELF_CORE)		+= elfcore.o