bpf: Add stub for btf_struct_access()

Add corresponding unimplemented stub for when CONFIG_BPF_SYSCALL=n

Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/4021398e884433b1fef57a4d28361bb9fcf1bd05.1662568410.git.dxu@dxuuu.xyz


Signed-off-by: Alexei Starovoitov <ast@kernel.org>