ctf: Use g_time_val_from_iso8601 instead of g_date_time_new_from_iso8601

ctf: Use g_time_val_from_iso8601 instead of g_date_time_new_from_iso8601

g_date_time_new_from_iso8601 was introduced in glib 2.56, which is quite
recent.  To support build with older glibs, change the code to use
g_time_val_from_iso8601 instead, introduced in glib 2.12.

The code in question only uses this function to validate that the passed
string is ISO8601-compliant, it doesn't use the resulting time value.

The problem of g_time_val_from_iso8601 is that its result is not year
2038-safe on 32-bit systems.  We can expect that by then, that function
will have been removed from glib, forcing us to update our code.

Change-Id: I2f4fdfce9020ecefcfb5d653a3885d343e49bd76
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1245
Reviewed-by: Michael Jeanson <mjeanson@efficios.com>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>