[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>