[S390] perf_counter build fix

Add PERF_COUNTER_INDEX_OFFSET define to fix this build bug:

kernel/perf_counter.c: In function 'perf_counter_index':
kernel/perf_counter.c:1889: error: 'PERF_COUNTER_INDEX_OFFSET' undeclared

Same fix as for FRV since s390 doesn't support hw counters.
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/include/asm/perf_counter.h

@@ -6,3 +6,5 @@
 
 static inline void set_perf_counter_pending(void) {}
 static inline void clear_perf_counter_pending(void) {}
+
+#define PERF_COUNTER_INDEX_OFFSET 0