rec: avoid implicit truncating cast of inception skew

rec: avoid implicit truncating cast of inception skew

Avoid coverity complaint:
store_truncates_time_t: A time_t value is stored in an integer with too few bits to accommodate it.
The expression sig.d_siginception - g_signatureInceptionSkew is cast to unsigned int.

Signed-off-by: Otto Moerbeek <otto.moerbeek@open-xchange.com>