tracing: Make function ‘ftrace_exports’ static

tracing: Make function ‘ftrace_exports’ static

In commit 478409dd683d ("tracing: Add hook to function tracing for other
subsystems to use"), a new function ‘ftrace_exports’ was added. Since
this function can be made static, make it so.

Silence the following warning triggered using W=1:

  kernel/trace/trace.c:2451:6: warning: no previous prototype for ‘ftrace_exports’ [-Wmissing-prototypes]

Link: http://lkml.kernel.org/r/20180516193012.25390-1-malat@debian.org
Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>