selftests/bpf: Temporarily disable dummy_struct_ops test on s390

Temporarily disable dummy_struct_ops test on s390.
The breakage is likely due to
commit 2cd3e377 ("x86/cfi,bpf: Fix bpf_struct_ops CFI").
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/DENYLIST.s390x

 # TEMPORARY
 # Alphabetical order
+dummy_st_ops/dummy_init_ret_value
+dummy_st_ops/dummy_init_ptr_arg
 exceptions				 # JIT does not support calling kfunc bpf_throw				       (exceptions)
 get_stack_raw_tp                         # user_stack corrupted user stack                                             (no backchain userspace)
 stacktrace_build_id                      # compare_map_keys stackid_hmap vs. stackmap err -2 errno 2                   (?)