devlink, rdma, tipc: properly define TARGETS without HAVE_MNL

devlink, rdma, tipc: properly define TARGETS without HAVE_MNL

Leaving a variable with a generic name such as TARGETS undefined would lead
to Make picking up its value from the environment. Avoid this by always
defining TARGETS in the Makefiles.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>