tracing: Rename trace/ftrace.h to trace/trace_events.h

The name "ftrace" really refers to the function hook infrastructure. It
is not about the TRACE_EVENT() macros. The file trace/ftrace.h was originally
written to be mostly focused toward the "ftrace" code (that in kernel/trace/)
but ended up being generic and used by perf and others.

Rename the file to be less confusing about what infrastructure it belongs to.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>