perf stat: Make function perf_stat_evsel_id_init static

perf stat: Make function perf_stat_evsel_id_init static

Function perf_stat_evsel_id_init() has global linkage but is only used
in util/stat.c. Make it static.

Signed-off-by: Thomas Richter <tmricht@linux.vnet.ibm.com>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Link: http://lkml.kernel.org/r/20180312103807.45069-2-tmricht@linux.vnet.ibm.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>