KVM: selftests: Build and link selftests/cgroup/lib into KVM selftests

libcgroup.o is built separately from KVM selftests and cgroup selftests,
so different compiler flags used by the different selftests will not
conflict with each other.

Signed-off-by: James Houghton <jthoughton@google.com>
Link: https://lore.kernel.org/r/20250508184649.2576210-7-jthoughton@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>