tracing: Fix trace_pid_list_free() kernel-doc

tracing: Fix trace_pid_list_free() kernel-doc

make C=1 reports:

kernel/trace/pid_list.c:458: warning: Function parameter or struct member 'pid_list' not described in 'trace_pid_list_free'

Add the missing parameter to the trace_pid_list_free() kernel-doc.

Link: https://lore.kernel.org/linux-trace-kernel/20240506-trace_pid_list_free-kdoc-v1-1-c70f0ae29144@quicinc.com
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>