rtla: use the definition for stdout fd when calling isatty()

Use the STDOUT_FILENO definition when testing whether the standard
output file descriptor refers to a terminal (for better redability).

Link: https://lore.kernel.org/20240813142338.376039-1-ezulian@redhat.com


Signed-off-by: Eder Zulian <ezulian@redhat.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>