selftest/ftrace: Move kprobe selftest function to separate compile unit

Move selftest function to its own compile unit so it can be compiled
with the ftrace cflags (CC_FLAGS_FTRACE) allowing it to be probed
during the ftrace startup tests.

Link: http://lkml.kernel.org/r/153294604271.32740.16490677128630177030.stgit@devbox

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>