Fix format-string warning

Cast size_t to (unsigned long) and print it with the %lu format
string, which is more portable than C99's explict %zu for size_t.

This truncates on Windows x64 but otherwise works on 32 and 64 bit
platforms. In practice the stats file is unlikely to be so large.

Change-Id: I0432b3acf85fc6ba4ad50640942e1ca4614b21cb