cmake: guard unittest_global_doublefree with WITH_LIBCEPHFS

cmake: guard unittest_global_doublefree with WITH_LIBCEPHFS

The unittest_global_doublefree test detects double-free issues in
global static variables when an executable links both librados and
libcephfs. However, it was incorrectly guarded by WITH_CEPHFS (which
controls the server-side cephfs components) instead of WITH_LIBCEPHFS
(which controls the client library it actually depends on).

This causes build failures when building with WITH_LIBCEPHFS enabled
but WITH_CEPHFS disabled:

```
    /usr/bin/ld: cannot find -lcephfs: No such file or directory
```

Change the guard to WITH_LIBCEPHFS to match the actual dependency.

While this scenario is uncommon (most users enable WITH_CEPHFS), the
fix correctly aligns the build logic with the test's dependencies.

Signed-off-by: Kefu Chai <k.chai@proxmox.com>