rtla: Add generated output files to gitignore

rtla: Add generated output files to gitignore

The rtla tool generates various output files during testing and
execution, including custom trace outputs and histogram data. These
files are artifacts of running the tool with different options and
should not be tracked in version control.

Add gitignore entries for custom_filename.txt, osnoise_irq_noise_hist.txt,
osnoise_trace.txt, and timerlat_trace.txt to prevent accidentally
committing these generated files. This aligns with the existing pattern
of ignoring build artifacts and generated headers like *.skel.h.

Signed-off-by: Wander Lairson Costa <wander@redhat.com>
Link: https://lore.kernel.org/r/20260106133655.249887-15-wander@redhat.com
Signed-off-by: Tomas Glozar <tglozar@redhat.com>