s390/perf: make cf_diag_csd static

Since there is really no reason for cf_diag_csd per cpu variable to be
globally visible make it static to avoid the following sparse warning:
arch/s390/kernel/perf_cpum_cf_diag.c:37:1: warning: symbol 'cf_diag_csd' was not declared. Should it be static?

Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>