s390/perf_events: compile only for CONFIG_64BIT

The whole hardware support is only available in zArch mode.
Fixes also this compile warning:

arch/s390/kernel/perf_cpum_cf.c: In function ‘cpumf_pmu_init’:
arch/s390/kernel/perf_cpum_cf.c:670:2: warning: left shift count >= width of type [enabled by default]

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/kernel/Makefile

@@ -48,8 +48,11 @@ obj-$(CONFIG_DYNAMIC_FTRACE)	+= ftrace.o
 obj-$(CONFIG_FUNCTION_GRAPH_TRACER) += ftrace.o
 obj-$(CONFIG_FTRACE_SYSCALLS)  += ftrace.o
 obj-$(CONFIG_CRASH_DUMP)	+= crash_dump.o
+
+ifdef CONFIG_64BIT
 obj-$(CONFIG_PERF_EVENTS)	+= perf_event.o perf_cpum_cf.o
-obj-$(CONFIG_64BIT)		+= runtime_instr.o cache.o
+obj-y				+= runtime_instr.o cache.o
+endif
 
 # Kexec part
 S390_KEXEC_OBJS := machine_kexec.o