bpf: Rebuild bpf.o for any dependency update

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 <[email protected]>
Cc: Alexei Starovoitov <[email protected]>
Cc: Daniel Borkmann <[email protected]>
Cc: David S. Miller <[email protected]>
Cc: Wang Nan <[email protected]>
Signed-off-by: David S. Miller <[email protected]>