shared: Remove circular dependency between image-policy.h and dissect-image.h

ImagePolicy can just be a forward declaration in dissect-image.h.

src/shared/mount-util.c

@@ -22,6 +22,7 @@
 #include "initrd-util.h"
 #include "label-util.h"
 #include "libmount-util.h"
+#include "log.h"
 #include "missing_syscall.h"
 #include "mkdir-label.h"
 #include "mount-util.h"