selftests/ftrace: Make a script checkbashisms clean

Make kprobe_ftrace.tc checkbashisms clean. Since
"grep function available_tracers" causes an error
on checkbashisms, fix it by explicitly escaping
with double-quotations.

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>