Build issue on Ubuntu 19.10

@pleiter reports

While reviewing D5.4, I made an attempt to build maestro-core (with MIO because of the Mero dependence) on Ubuntu 19.10 and had an issue, because built failed with the following error message:
make[2]: Entering directory '/home/pleiter/hpc/prj/fethpc/maestro/src/maestro-core/build/transport'
  CC       libmaestro_transport_la-gfs.lo
In file included from ../../git/transport/transport.h:40,
                 from ../../git/transport/gfs.c:2:
../../git/transport/../protocols/mstro_pool.pb-c.h:7:10: fatal error: protobuf-c/protobuf-c.h: No such file or directory
    7 | #include <protobuf-c/protobuf-c.h>
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~
compilation terminated.
make[2]: *** [Makefile:464: libmaestro_transport_la-gfs.lo] Error 1
make[2]: Leaving directory '/home/pleiter/hpc/prj/fethpc/maestro/src/maestro-core/build/transport'
I only found time today for looking into this. Inspecting the failed command revealed that "../../git/deps/protobuf-c" was not in the include path such that "../../git/deps/protobuf-c/protobuf-c/protobuf-c.h" was not found.

maestro-core.diff