www/receiptomatic-www.c: Use CLOCK_MONOTONIC in init_clear_session_timer()

Previously we were using CLOCK_REALTIME, however this can be affected by
changes to the system clock. While its use here doesn't really dictate
high timing precision, CLOCK_MONOTONIC is the right thing to use as it
won't be changed after system boot.

Signed-off-by: Andrew Clayton <andrew@digital-domain.net>