tracing: Convert trace_destroy_fields() to static

trace_destroy_fields() is not used outside of the file. It can be
a static function.

Link: http://lkml.kernel.org/r/513D842A.2000907@huawei.com

Signed-off-by: zhangwei(Jovi) <jovi.zhangwei@huawei.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

kernel/trace/trace_events.c

@@ -158,7 +158,7 @@ static int trace_define_common_fields(void)
 	return ret;
 }
 
-void trace_destroy_fields(struct ftrace_event_call *call)
+static void trace_destroy_fields(struct ftrace_event_call *call)
 {
 	struct ftrace_event_field *field, *next;
 	struct list_head *head;