tools/power turbostat: Fix PMT mmaped file size rounding

This (the old code) is just not how you round up to a page size.
Noticed on a recent Intel platform. Previous ones must have been
reporting sizes already aligned to a page and so the bug was missed when
testing.

Fixes: f0e4ed75 ("tools/power turbostat: Add early support for PMT counters")
Signed-off-by: Patryk Wlazlyn <patryk.wlazlyn@linux.intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>