image_types: Set SOURCE_DATE_EPOCH for squashfs

We want to use the reproducible timestamp for all of the files that is
set rootfs-postcommands.bbclass, derived from
REPRODUCIBLE_TIMESTAMP_ROOTFS. Without this, we use a hardcoded time
that is built into the squashfs sources.

Signed-off-by: William A. Kennington III <wak@google.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>