src: silence compiler warning in node_report.cc

Currently the following compiler warnings is generated:

../src/node_report.cc:778:43:
warning: format specifies type 'unsigned long' but the argument has
type 'rlim_t' (aka 'unsigned long long') [-Wformat]
        snprintf(buf, sizeof(buf), "%lu", limit.rlim_max);
                                    ~~~   ^~~~~~~~~~~~~~
                                    %llu
1 warning generated.

This commit changes the format specifier to $llu.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines