[S390] ftrace: function tracer backend for s390

This implements just the basic function tracer (_mcount) backend for s390.
The dynamic variant will come later.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
index 90ba5d3..e6efc12 100644
--- a/arch/s390/Kconfig
+++ b/arch/s390/Kconfig
@@ -74,6 +74,7 @@
 config S390
 	def_bool y
 	select USE_GENERIC_SMP_HELPERS if SMP
+	select HAVE_FUNCTION_TRACER
 	select HAVE_OPROFILE
 	select HAVE_KPROBES
 	select HAVE_KRETPROBES