bpf: Fix cold build of test_progs-no_alu32

This object lives inside the trunner output dir,
i.e. tools/testing/selftests/bpf/no_alu32/btf_data.o

At some point it gets copied into the parent directory during another
part of the build, but that doesn't happen when building
test_progs-no_alu32 from clean.

Signed-off-by: Brendan Jackman <jackmanb@google.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Link: https://lore.kernel.org/bpf/20201203120850.859170-1-jackmanb@google.com