examples/elementary: drop generated.h from SOURCES

examples/elementary: drop generated.h from SOURCES

Remove codegen_example_generated.h from codegen_example_SOURCES
and let it only on nodist_codegen_example_SOURCES and
on BUILT_SOURCES.

Also add dependency between codegen_example.c
and codegen_example_generated.h since it's required
to compile.

Avoid the following build error:
  CODEGEN      codegen_example_generated.c
  codegen_example.c:26:39: fatal error: codegen_example_generated.h:
      No such file or directory
  compilation terminated.
  Makefile:4960: recipe for target 'codegen_example.o' failed