tracing: Make tracer_tracing_{off,on,is_on}() static

I have patches that will use tracer_tracing_on/off/is_on() in other
files, but as they are not ready to be merged yet, and Fengguang Wu's
sparse scripts pointed out that these functions were not declared
anywhere, I'll make them static for now.

When these functions are required to be used elsewhere, I'll remove
the static then.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>