Update to support gs/kernel/device-modules as an external module

* Include the Makefile.include from gs/kernel/device-modules which
  updates EXTRA_CFLAGS and EXTRA_SYMBOLS to be able to include headers
  from there as well as link against modules under from there.
* Update BUILD.bazel to support these changes as well.

Signed-off-by: Will McVicker <willmcvicker@google.com>
Change-Id: I72ba9a23e7e204a6689f080095b4eb0c401e9709