selftests/bpf: Fix progs/find_vma_fail1.c build error.

The commit 11e456ca ("selftests/bpf: Fix compilation errors: Assign a value to a constant")
fixed the issue cleanly in bpf-next.
This is an alternative fix in bpf tree to avoid merge conflict between bpf and bpf-next.

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