Fix unused function error for user build

```
system/core/fs_mgr/fs_mgr_overlayfs.cpp:83:6: error: unused function
'fs_mgr_is_dsu_running' [-Werror,-Wunused-function]
```

Bug: 172405271
Bug: 165925766
Test: build aosp_bonito-user, no warning/error
Change-Id: I3206e5bd13658ab0b7f5826e8e1c206c8df65684