Fix includes for inclusion as an external project

[Shillaker]

When using Faabric as an external project it can either be added with add_subdirectory in which case we need the public target_include_directories or as an installed library in which case the transitive header includes should still work.

[csegarragonz]

Closes #43

[csegarragonz]

@Shillaker I was trying to rebuild this myself, but the examples build seems to fail (albeit the tests "pass"). In fact, if you check the latest action, under the tab Build Examples you will see that the build step for the server fails.

To replicate just run the steps in the workflow file:

inv dev.cmake --shared
inv dev.cc faabric --shared
inv dev.install faabric --shared
inv examples

To workaround this issue, we need to link with protobuf in examples/CMakeLists. The following works for me:

target_link_libraries(${example_name}
        ${FAABRIC_LIBS}
        pistache
        protobuf
        )

PS: I think this is not completely right, just a workaround that happens to work in my machine.

I will now have a look into why the tests actually pass.