bpf: Rebuild bpf.o for any dependency update

This is needed to force a rebuild of bpf.o when one of its dependencies
(e.g. uapi/linux/bpf.h) is updated.

Add a phony target.

Signed-off-by: Mickaël Salaün <mic@digikod.net>
Cc: Alexei Starovoitov <ast@fb.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: David S. Miller <davem@davemloft.net>
Cc: Wang Nan <wangnan0@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>