selftests/bpf: fix Makefile for cgroup_helpers.c

The current selftests Makefile construct result in cgroup_helpers.c
gets compiled together with all the TEST_GEN_PROGS. And it also result
in invoking the libbpf Makefile two times (tools/lib/bpf).

These issues were introduced in commit 9d1f1594 ("bpf: move
cgroup_helpers from samples/bpf/ to tools/testing/selftesting/bpf/").

The only test program that requires the cgroup helpers is 'test_dev_cgroup'.

Thus, create a make target $(OUTPUT)/test_dev_cgroup that extend[1]
the 'prerequisite' for the 'stem' %-style pattern in ../lib.mk,
for this particular test program.

Reviewers notice the make-rules in tools/testing/selftests/lib.mk
differ from the normal kernel kbuild rules, and it is practical
to use 'make -p' to follow how these 'Implicit/static pattern stem'
gets expanded.

[1] https://www.gnu.org/software/make/manual/html_node/Static-Usage.html

Fixes: 9d1f1594

 ("bpf: move cgroup_helpers from samples/bpf/ to tools/testing/selftesting/bpf/")
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>