Fix bug: increment the value of pmcstat_npmcs instead of moving pointer
to the next int position.

Bug was introduced in r324959 ("Extract a set of pmcstat functions and
interfaces to the new internal library -- libpmcstat.")

This fixes pmcstat top mode (-T) operation.
Example: pmcstat -n1 -S clock.hard -T

Reported by: Peter Holm <peter@holm.cc>
Sponsored by: DARPA, AFRL