common: fix a FTBFS in Journald.cc when building without systemd

common: fix a FTBFS in Journald.cc when building without systemd

This came from Josh Salomon:

```
/home/jsalomon/src/prim_score/src/common/Journald.h:93:24: error: ‘SubsystemMap’ does not name a type
   93 |   JournaldLogger(const SubsystemMap *) {}
      |                        ^~~~~~~~~~~~
/home/jsalomon/src/prim_score/src/common/Journald.h:94:23: error: ‘Entry’ does not name a type
   94 |   int log_entry(const Entry &) {
      |
```

Signed-off-by: Radosław Zarzyński <rzarzyns@redhat.com>