tracing: Remove unused argument from "ring_buffer_time_stamp()

The "cpu" parameter is not being used by the function.

Link: https://lkml.kernel.org/r/20210329130331.199402-1-y.karadz@gmail.com



Signed-off-by: Yordan Karadzhov (VMware) <y.karadz@gmail.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>