rtla: Define __NR_sched_setattr for LoongArch

When executing "make -C tools/tracing/rtla" on LoongArch, there exists
the following error:

  src/utils.c:237:24: error: '__NR_sched_setattr' undeclared

Just define __NR_sched_setattr for LoongArch if not exist.

Link: https://lore.kernel.org/20250422074917.25771-1-yangtiezhu@loongson.cn


Reported-by: Haiyong Sun <sunhaiyong@loongson.cn>
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>