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

The commit 11e456c ("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>

Author: Alexei Starovoitov

tools/testing/selftests/bpf/progs/find_vma_fail1.c

@@ -2,6 +2,7 @@
 /* Copyright (c) 2021 Facebook */
 #include "vmlinux.h"
 #include <bpf/bpf_helpers.h>
+#define vm_flags vm_start
 
 char _license[] SEC("license") = "GPL";