core: Break circular dependency between unit.h and cgroup.h

src/core/cgroup.h

@@ -127,7 +127,8 @@ typedef enum CGroupIPAccountingMetric {
         _CGROUP_IP_ACCOUNTING_METRIC_INVALID = -1,
 } CGroupIPAccountingMetric;
 
-#include "unit.h"
+typedef struct Unit Unit;
+typedef struct Manager Manager;
 
 void cgroup_context_init(CGroupContext *c);
 void cgroup_context_done(CGroupContext *c);

src/core/dbus-cgroup.h

@@ -9,6 +9,7 @@
 
 #include "sd-bus.h"
 
+#include "unit.h"
 #include "cgroup.h"
 
 extern const sd_bus_vtable bus_cgroup_vtable[];