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