selftests/bpf: make directory prerequisites order-only

When directories are used as prerequisites in Makefiles, they can cause
a lot of unnecessary rebuilds, because a directory is considered changed
whenever a file in this directory is added, removed or modified.

If the only thing a target is interested in is the existence of the
directory it depends on, which is the case for selftests/bpf, this
directory should be specified as an order-only prerequisite: it would
still be created in case it does not exist, but it would not trigger a
rebuild of a target in case it's considered changed.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>