bpf: Update selftests for local_storage to use vmlinux.h

With the fixing of BTF pruning of embedded types being fixed, the test
can be simplified to use vmlinux.h

Signed-off-by: KP Singh <kpsingh@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20201106103747.2780972-8-kpsingh@chromium.org