selftests/bpf: Install all required files to run selftests

When installing the selftests using
"make -C tools/testing/selftests install", we need to make sure
all the required files to run the selftests are installed. Let's
make sure this is the case.

Signed-off-by: Daan De Meyer <daan.j.demeyer@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20221205131618.1524337-2-daan.j.demeyer@gmail.com