xen/tracing: it looks like we wanted CONFIG_FTRACE

Apparently we wanted CONFIG_FTRACE rather the CONFIG_FUNCTION_TRACER.
Reported-by: Sander Eikelenboom <linux@eikelenboom.it>
Tested-by: Sander Eikelenboom <linux@eikelenboom.it>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

arch/x86/xen/Makefile

@@ -15,7 +15,7 @@ obj-y		:= enlighten.o setup.o multicalls.o mmu.o irq.o \
 			grant-table.o suspend.o platform-pci-unplug.o \
 			p2m.o
 
-obj-$(CONFIG_FUNCTION_TRACER) += trace.o
+obj-$(CONFIG_FTRACE) += trace.o
 
 obj-$(CONFIG_SMP)		+= smp.o
 obj-$(CONFIG_PARAVIRT_SPINLOCKS)+= spinlock.o