selftests/ftrace: Add required dependency for kprobe tests

kprobe_args_{char,string}.tc are using available_filter_functions file
which is provided by function tracer. Thus if function tracer is disabled,
these tests are failed on recent kernels because tracefs_create_dir is
not raised events by adding a dynamic event.
Add available_filter_functions to requires line.

Fixes: 7c1130ea ("test: ftrace: Fix kprobe test for eventfs")
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>