samples/bpf: fix warning with ignored-attributes

Currently, compiling the bpf programs will result the warning with the
ignored attribute as follows. This commit fixes the warning by adding
cf-protection option.

    In file included from ./arch/x86/include/asm/linkage.h:6:
    ./arch/x86/include/asm/ibt.h:77:8: warning: 'nocf_check' attribute ignored; use -fcf-protection to enable the attribute [-Wignored-attributes]
    extern __noendbr u64 ibt_save(bool disable);
           ^
    ./arch/x86/include/asm/ibt.h:32:34: note: expanded from macro '__noendbr'
                                       ^

Signed-off-by: Daniel T. Lee <danieltimlee@gmail.com>
Link: https://lore.kernel.org/r/20230818090119.477441-2-danieltimlee@gmail.com


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