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

tools/testing/selftests/bpf/config

@@ -34,3 +34,4 @@ CONFIG_NET_MPLS_GSO=m
 CONFIG_MPLS_ROUTING=m
 CONFIG_MPLS_IPTUNNEL=m
 CONFIG_IPV6_SIT=m
+CONFIG_BPF_JIT=y