EDAC, i10nm: make skx_common.o a separate module

Commit 598afa05 ("kbuild: warn objects shared among multiple modules")
was added to track down cases where the same object is linked into
multiple modules. This can cause serious problems if some modules are
builtin while others are not.

That test triggers this warning:

scripts/Makefile.build:236: drivers/edac/Makefile: skx_common.o is added to multiple modules: i10nm_edac skx_edac

Make this a separate module instead.

[Tony: Added more background details to commit message]

Fixes: d4dc89d0 ("EDAC, i10nm: Add a driver for Intel 10nm server processors")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Tony Luck <tony.luck@intel.com>
Link: https://lore.kernel.org/all/20240529095132.1929397-1-arnd@kernel.org/