ftrace: move sysctl_ftrace_enabled to ftrace.c

This moves ftrace_enabled to trace/ftrace.c.

We move sysctls to places where features actually belong to improve
the readability of the code and reduce the risk of code merge conflicts.
At the same time, the proc-sysctl maintainers do not want to know what
sysctl knobs you wish to add for your owner piece of code, we just care
about the core logic.

Signed-off-by: Wei Xiao <xiaowei66@huawei.com>
Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>