ftrace: remove static from function tracer functions

Impact: clean up

After reorganizing the functions in trace.c and trace_function.c,
they no longer need to be in global context. This patch makes the
functions and one variable into static.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>