selftests/bpf: add config fragment BPF_JIT

When running test_kmod.sh the following shows up

 # sysctl cannot stat /proc/sys/net/core/bpf_jit_enable No such file or directory
 cannot: stat_/proc/sys/net/core/bpf_jit_enable #
 # sysctl cannot stat /proc/sys/net/core/bpf_jit_harden No such file or directory
 cannot: stat_/proc/sys/net/core/bpf_jit_harden #

Rework to enable CONFIG_BPF_JIT to solve "No such file or directory"

Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>