tracing: Rename ftrace_define_fields_##call() to trace_event_define_fields_##call()

The name "ftrace" really refers to the function hook infrastructure. It
is not about the trace_events. The function ftrace_define_fields_##call()
is used to define how to process the trace_event fields. It has nothing to
do with function tracing. Rename it.

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