perf script: Use strtok_r() when parsing output field list

Just avoiding non-reentrant functions.

Cc: David Ahern <dsahern@gmail.com>
Link: http://lkml.kernel.org/n/tip-eqytykipd74epzl9aexvppcg@git.kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>