selftests/bpf: Don't include .d files on make clean

Ignore generated %.test.o dependencies when make goal is clean or
docs-clean.

Signed-off-by: Ihor Solodrai <ihor.solodrai@pm.me>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/all/oNTIdax7aWGJdEgabzTqHzF4r-WTERrV1e1cNaPQMp-UhYUQpozXqkbuAlLBulczr6I99-jM5x3dxv56JJowaYBkm765R9Aa9kyrVuCl_kA=@pm.me
Link: https://lore.kernel.org/bpf/K69Y8OKMLXBWR0dtOfsC4J46-HxeQfvqoFx1CysCm7u19HRx4MB6yAKOFkM6X-KAx2EFuCcCh_9vYWpsgQXnAer8oQ8PMeDEuiRMYECuGH4=@pm.me