Fix GCC-7 warning on buffer size

ticket.c: In function ‘process_tickets’:
ticket.c:470:44: warning: ‘%s’ directive output may be truncated writing up to 63 bytes into a region of size 53 [-Wformat-truncation=]

snprintf(buff, sizeof(buff), "granted to %s",
                                         ^~

ticket.c:470:3: note: ‘snprintf’ output between 12 and 75 bytes into a destination of size 64

snprintf(buff, sizeof(buff), "granted to %s",
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 site_string(tk->leader));
 ~~~~~~~~~~~~~~~~~~~~~~~~