tracing/events: enable code with EVENT_TRACING not EVENT_TRACER

The CONFIG_EVENT_TRACER is the way to turn on event tracing when no
other tracing has been configured. All code to get enabled should
depend on CONFIG_EVENT_TRACING. That is what is enabled when TRACING
(or CONFIG_EVENT_TRACER) is selected.

This patch enables the include/trace/ftrace.h file when
CONFIG_EVENT_TRACING is enabled.

[ Impact: fix warning in event tracer selftest ]

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