perf-event-open-sys: fix argument type for ioctls::PERIOD

[mbyzhang]

PERF_EVENT_IOC_PERIOD ioctl is expecting a pointer to a u64 rather than a plain u64*. This commit fixes that.

*: in perf_event_open(2) manpage

       PERF_EVENT_IOC_PERIOD
              This updates the overflow period for the event.

              Since Linux 3.7 (on ARM) and Linux 3.14 (all other
              architectures), the new period takes effect immediately.
              On older kernels, the new period did not take effect until
              after the next overflow.

              The argument is a pointer to a 64-bit value containing the
              desired new period.

              Prior to Linux 2.6.36, this ioctl always failed due to a
              bug in the kernel.

[mbyzhang]

V2: fixed the CI failure