The `helloworld-simple-agent.c` source file includes `helloworld-tick-board.h`
which in turn includes the generated
`org.apertis.HelloWorld.SimpleAgent.TickBoard.h` header file.

However, `gdbus-gen` is only called to generate the TickBoard header when
building the `libhelloworld-simple-agent-generated.la` internal library, while
`helloworld-simple-agent.c` is part of
`libhelloworld-simple-agent-internal.la`.

While this used to work by chance, newer Make chooses a different ordering
leading to the error below:

      CC       helloworld-simple-agent/main.o
      CC       helloworld-simple-agent/helloworld-simple-agent.lo
    In file included from helloworld-simple-agent/helloworld-simple-agent.c:12:
    helloworld-simple-agent/helloworld-tick-board.h:17:10: fatal error: org.apertis.HelloWorld.SimpleAgent.TickBoard.h: No such file or directory
     #include "org.apertis.HelloWorld.SimpleAgent.TickBoard.h"
              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    compilation terminated.

To avoid the issue, add the generated source files to the automake
`BUILT_SOURCES` variable so they get generated before the actual
compilation begins.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>