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: Fathi Boudra <fathi.boudra@linaro.org>
Tested-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>