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>