fs-sink-trace.c: lttng_validate_datetime(): ignore deprecated decl.

Disable `deprecated-declarations` compiler warnings for
lttng_validate_datetime() because we're using `GTimeVal` and
g_time_val_from_iso8601() which are deprecated since GLib 2.56
(Babeltrace supports older versions too).

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I4da3bda550b3fd7d7ede13ff3b78ec78ad6f7839
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2121
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>