Makefile: Add missing dep on the generated DBus header file

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.