Fix statistics retention default value.

60s * 60m * 24h * 31d * 12m * 5 != 5 years
60s * 60m * 365d * 5 != 5 years

But the second value is much closer then using 12 months with
31 days of which 4 months have 30 days and one has either 28
e.g. 3 days less or at best 29 days so 2 days less. When multiplied
with 5 years this mean you calculate something 30+ days to much.

src/dird/dird_conf.c

@@ -138,7 +138,7 @@ static RES_ITEM dir_items[] = {
    { "tlskey", store_dir, ITEM(res_dir.tls_keyfile), 0, 0, NULL },
    { "tlsdhfile", store_dir, ITEM(res_dir.tls_dhfile), 0, 0, NULL },
    { "tlsallowedcn", store_alist_str, ITEM(res_dir.tls_allowed_cns), 0, 0, NULL },
-   { "statisticsretention", store_time, ITEM(res_dir.stats_retention), 0, ITEM_DEFAULT, "160704000" /* 5 years */ },
+   { "statisticsretention", store_time, ITEM(res_dir.stats_retention), 0, ITEM_DEFAULT, "157680000" /* 5 years */ },
    { "verid", store_str, ITEM(res_dir.verid), 0, 0, NULL },
    { "optimizeforsize", store_bool, ITEM(res_dir.optimize_for_size), 0, ITEM_DEFAULT, "true" },
    { "optimizeforspeed", store_bool, ITEM(res_dir.optimize_for_speed), 0, ITEM_DEFAULT, "false" },