selftests/bpf: Allow veristat compile standalone

Veristat is synced into the standalone repo, where it compiles without
kernel private dependencies. This patch fixes compilation errors in
standalone veristat.

Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20250702175622.358405-1-mykyta.yatsenko5@gmail.com