Parallel build fails

[kkaempf]

Describe the issue

make clean; make -j 12 fails with

In file included from ./piscsi/piscsi_executor.cpp:11:
./shared/protobuf_util.h:17:10: fatal error: generated/piscsi_interface.pb.h: No such file or directory
   17 | #include "generated/piscsi_interface.pb.h"
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
compilation terminated.
make: *** [Makefile:170: obj/piscsi_executor.o] Error 1
make: *** Waiting for unfinished jobs....
In file included from ./devices/disk.h:20,
                 from ./piscsi/piscsi_image.cpp:10:
./devices/disk_cache.h:18:10: fatal error: generated/piscsi_interface.pb.h: No such file or directory
   18 | #include "generated/piscsi_interface.pb.h"
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
compilation terminated.