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>

tools/testing/selftests/ftrace/test.d/kprobe/kprobe_args_char.tc

 #!/bin/sh
 # SPDX-License-Identifier: GPL-2.0
 # description: Kprobe event char type argument
-# requires: kprobe_events
+# requires: kprobe_events available_filter_functions
 
 case `uname -m` in
 x86_64)

tools/testing/selftests/ftrace/test.d/kprobe/kprobe_args_string.tc

 #!/bin/sh
 # SPDX-License-Identifier: GPL-2.0
 # description: Kprobe event string type argument
-# requires: kprobe_events
+# requires: kprobe_events available_filter_functions
 
 case `uname -m` in
 x86_64)