selftests/bpf: add config fragment CONFIG_FTRACE_SYSCALLS

CONFIG_FTRACE_SYSCALLS=y is required for get_cgroup_id_user test case
this test reads a file from debug trace path
/sys/kernel/debug/tracing/events/syscalls/sys_enter_nanosleep/id

Signed-off-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>