xen/xenbus: Use *-y instead of *-objs in Makefile

xen/xenbus: Use *-y instead of *-objs in Makefile

*-objs suffix is reserved rather for (user-space) host programs while
usually *-y suffix is used for kernel drivers (although *-objs works
for that purpose for now).

Let's correct the old usages of *-objs in Makefiles.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20240508152658.1445809-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Juergen Gross <jgross@suse.com>