add check for CONFIG_BPF_JIT and disable warning on jit hardening in the case it is disabled

CONFIG_BPF_JIT is the JIT engine hardened by CONFIG_GRKERNSEC_JIT_HARDEN and a prerequisite for the hardening option.
My pull request changes the output of checksec.sh in the case that CONFIG_BPF_JIT is disabled to green and tells the user that JIT is disabled anyways.
This way there shouldn't be much irritations about a disabled (unnecessary) security option.