rtla/utils: Fix session duration parsing

Use gmtime to format the duration time. This avoids problems when the
system uses local time different of Pisa's Local Time.

Link: https://lkml.kernel.org/r/a2f0a37bc006c2561bb8ecd871cd70532b4a9f2d.1643990447.git.bristot@kernel.org

Fixes: b1696371

 ("rtla: Helper functions for rtla")
Cc: Daniel Bristot de Oliveira <bristot@kernel.org>
Cc: linux-kernel@vger.kernel.org
Cc: linux-trace-devel@vger.kernel.org
Signed-off-by: Daniel Bristot de Oliveira <bristot@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>