processes plugin: fix build warning

processes plugin: fix build warning

  CC       src/processes.lo
src/processes.c: In function ‘ps_read’:
src/processes.c:823:58: warning: ‘%s’ directive output may be truncated writing up to 255 bytes into a region of size between 32 and 51 [-Wformat-truncation=]
     snprintf(filename, sizeof(filename), "/proc/%li/task/%s/status", ps->id,
                                                          ^~
src/processes.c:823:5: note: ‘snprintf’ output between 21 and 295 bytes into a destination of size 64
     snprintf(filename, sizeof(filename), "/proc/%li/task/%s/status", ps->id,
     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
               tpid);
               ~~~~~

In practice the buffer is more than large enough, since all we substitute are process ids, but gcc can't know that.