tracing: Move the tracing_on/off() declarations into CONFIG_TRACING

tracing: Move the tracing_on/off() declarations into CONFIG_TRACING

The tracing_on/off() declarations were under CONFIG_RING_BUFFER, but
the functions are now only defined under CONFIG_TRACING as they are
specific to ftrace and not the ring buffer.

But the declarations were still defined under the ring buffer and
this caused the build to fail when CONFIG_RING_BUFFER was set but
CONFIG_TRACING was not.

Reported-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>