x86: fix build race when generating temporary object files (take 2)

x86: fix build race when generating temporary object files (take 2)

The original commit wasn't quite sufficient: Emptying DEPS is helpful
only when nothing will get added to it subsequently. xen/Rules.mk will,
after including the local Makefile, amend DEPS by dependencies for
objects living in sub-directories though. For the purpose of suppressing
dependencies of the makefiles on the .*.d2 files (and thus to avoid
their re-generation) it is, however, not necessary at all to play with
DEPS. Instead we can override DEPS_INCLUDE (which generally is a late-
expansion variable).

Fixes: 761bb575ce97 ("x86: fix build race when generating temporary object files")
Signed-off-by: Jan Beulich <jbeulich@suse.com>