ftrace: Always inline ftrace_hash_empty() helper function

The ftrace_hash_empty() function is a simple test:

	return !hash || !hash->count;

But gcc seems to want to make it a call. As this is in an extreme
hot path of the function tracer, there's no reason it needs to be
a call. I only wrote it to be a helper function anyway, otherwise
it would have been inlined manually.

Force gcc to inline it, as it could have also been a macro.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>