[S390] ftrace: build ftrace.o when CONFIG_FTRACE_SYSCALLS is set for s390

Trying to build a s390x kernel with CONFIG_FTRACE_SYSCALLS will fail
because ftrace.o is not built/linked.
Acked-by: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Aristeu Rozanski <aris@redhat.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/kernel/Makefile

@@ -44,6 +44,7 @@ obj-$(CONFIG_KPROBES)		+= kprobes.o
 obj-$(CONFIG_FUNCTION_TRACER)	+= $(if $(CONFIG_64BIT),mcount64.o,mcount.o)
 obj-$(CONFIG_DYNAMIC_FTRACE)	+= ftrace.o
 obj-$(CONFIG_FUNCTION_GRAPH_TRACER) += ftrace.o
+obj-$(CONFIG_FTRACE_SYSCALLS)  += ftrace.o
 
 # Kexec part
 S390_KEXEC_OBJS := machine_kexec.o crash.o