selftests/bpf: Add uprobe_multi to gen_tar target

The uprobe_multi program was not picked up for the gen_tar target. Fix
by adding it to TEST_GEN_FILES.

Signed-off-by: Björn Töpel <bjorn@rivosinc.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/bpf/20231004122721.54525-4-bjorn@kernel.org