samples: bpf: Don't fail for a missing VMLINUX_BTF when VMLINUX_H is provided

samples/bpf build currently always fails if it can't generate
vmlinux.h from vmlinux, even when vmlinux.h is directly provided by
VMLINUX_H variable, which makes VMLINUX_H pointless.
Only fails when neither method works.

Fixes: 384b6b3b

 ("samples: bpf: Add vmlinux.h generation support")
Reported-by: CKI Project <cki-project@redhat.com>
Reported-by: Veronika Kabatova <vkabatov@redhat.com>
Signed-off-by: Jerome Marchand <jmarchan@redhat.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220507161635.2219052-1-jmarchan@redhat.com