tracing: Fix comments about trace/ftrace.h

tracing: Fix comments about trace/ftrace.h

commit 81de6176caa31067f47e2e0987aaf3909bff766a moved tracepoint's ftrace
creation into include/trace/ftrace.h and trace/define_trace.h was deleted
as a result. However some comment info does not adapt to the change, which
is such a misguiding when reading related code.

This patch fix this by moving trace/trace_events.h to <trace/events/XXX.h>,
since tracepoint headers have already been moved to tarce/events/.

Link: http://lkml.kernel.org/r/1425419298-61941-1-git-send-email-houpengyang@huawei.com
Signed-off-by: Hou Pengyang <houpengyang@huawei.com>
[ Pulled from the archeological digging of my INBOX ]
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>