pc-bios/s390-ccw: Generate and include dependency files in the Makefile

pc-bios/s390-ccw: Generate and include dependency files in the Makefile

The Makefile of the s390-ccw bios does not handle dependencies of the
*.c files from the headers yet, so that you often have to run a "make
clean" to get the build right when one of the headers has been changed.
Let's make sure that we generate and include dependency files for all
*.c files now to avoid this problem in the future.

Acked-by: Cornelia Huck <cohuck@redhat.com>
Message-Id: <20200630142955.7662-1-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>