el7: cgroup_info_init() fails in CI with ubuntu-latest

[grondo]

The unit tests for the pidinfo class fail in the el7 builder when ubuntu-latest is used as the distro for the builder.

The problem appears to be that the host system is using cgroup2fs (new style cgroups), but the CGROUP2_SUPER_MAGIC define doesn't exist in el7, so cgroup_info_init() fails to detect cgroup2, resulting in a failure in pid_info_create().

Probably the best solution is define CGROUP2_SUPER_MAGIC manually when the define isn't found in a header.