Xen: fix cpu_hotplug build when !CONFIG_SMP

Compile cpu_hotplug.c conditionally on CONFIG_HOTPLUG_CPU
Signed-off-by: NAlex Nixon <alex.nixon@citrix.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

drivers/xen/Makefile

-obj-y	+= grant-table.o features.o events.o manage.o cpu_hotplug.o
+obj-y	+= grant-table.o features.o events.o manage.o
 obj-y	+= xenbus/
+obj-$(CONFIG_HOTPLUG_CPU)	+= cpu_hotplug.o
 obj-$(CONFIG_XEN_XENCOMM)	+= xencomm.o
 obj-$(CONFIG_XEN_BALLOON)	+= balloon.o