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

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

@@ -3,7 +3,7 @@
 # description: Kprobe dynamic event with function tracer
 
 [ -f kprobe_events ] || exit_unsupported # this is configurable
-grep function available_tracers || exit_unsupported # this is configurable
+grep "function" available_tracers || exit_unsupported # this is configurable
 
 # prepare
 echo nop > current_tracer