drm/sched: Fix preprocessor guard

When writing the header guard for gpu_scheduler_trace.h, a typo,
apparently, occurred.

Fix the typo and document the scope of the guard.

Fixes: 353da3c5 ("drm/amdgpu: add tracepoint for scheduler (v2)")
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Signed-off-by: Philipp Stanner <phasta@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20250218124149.118002-2-phasta@kernel.org