stat: fix printf format specifier

stat: fix printf format specifier

54c05828da413f6b4f9eec800a30624c453a4d90 introduced the printing of the
number of samples but on 64 bit macOS with clang the format specifier
generates warnings like:

stat.c:502:4: warning: format specifies type 'unsigned long' but the argument has type 'uint64_t'
      (aka 'unsigned long long') [-Wformat]
                        (&ts->bw_stat[ddir])->samples);
                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Fix this by using the PRIu64 macro.

Signed-off-by: Sitsofe Wheeler <sitsofe@yahoo.com>