selftests/bpf: Fix a compile error for BPF_F_BPRM_SECUREEXEC

When CONFIG_BPF_LSM is not configured, running bpf selftesting will show
BPF_F_BPRM_SECUREEXEC undefined error for bprm_opts.c.

The problem is that bprm_opts.c includes vmliunx.h. The vmlinux.h is
generated by "bpftool btf dump file ./vmlinux format c". On the other
hand, BPF_F_BPRM_SECUREEXEC is defined in include/uapi/linux/bpf.h
and used only in bpf_lsm.c. When CONFIG_BPF_LSM is not set, bpf_lsm
will not be compiled, so vmlinux.h will not include definition of
BPF_F_BPRM_SECUREEXEC.

Ideally, we want to compile bpf selftest regardless of the configuration
setting, so change the include file from vmlinux.h to bpf.h.

Signed-off-by: Jiang Wang <jiang.wang@bytedance.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Song Liu <songliubraving@fb.com>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20201224011242.585967-1-jiang.wang@bytedance.com