Makefile: Add missing dependency of '.depend' on 'oclobj.h'

Makefile: Add missing dependency of '.depend' on 'oclobj.h'

Without the change parallel build occasionally fails as:

    $ make --shuffle
    ...
    gcc ... -c common/opencl.c -o common/opencl-8.o ...
    common/opencl.c:116:10: fatal error: common/oclobj.h: No such file or directory
      116 | #include "common/oclobj.h"
          |          ^~~~~~~~~~~~~~~~~

Best reproducible with `make --shuffle` mode:
   https://savannah.gnu.org/bugs/index.php?62100

This happens because `common/oclobj.h` is an autogenerated file.
Normally `.depend` would contain this autogenerated dependency.
But nothing forces `common/oclobj.h` to be generated.

The change moves dependency of $(GENERATED) from final binaries
to `.depend` itself:

    .depend: $(GENERATED)