selftests: ftrace: add CONFIG_KPROBES=y to the config fragment

ftrace/kprobe tests require kprobes events. Enable kprobes to run these
tests.
Signed-off-by: NFathi Boudra <fathi.boudra@linaro.org>
Tested-by: NNaresh Kamboju <naresh.kamboju@linaro.org>
Signed-off-by: NShuah Khan <shuahkh@osg.samsung.com>

tools/testing/selftests/ftrace/config

+CONFIG_KPROBES=y
 CONFIG_FTRACE=y