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>