Make process.o depend on process.amd64.s

Make process.o depend on process.amd64.s

Some versions of the rule for this target use the amd64 flavor
of the source file, so it should be listed as a dependency.

This is a 1.6-specific change; the relevant dependency was added
on master as part of change #5450, which is a much larger change.

Change-Id: I5eb392ccd6b059f6f2344398bd697a5096a1bd77
Reviewed-on: http://gerrit.openafs.org/10382
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>