Arm32: prune (again) ld warning about mismatched wchar_t sizes

The name change (stub.c -> common-stub.c) rendered the earlier
workaround (commit a4d4c541f5 ["xen/arm32: avoid EFI stub wchar_t size
linker warning"]) ineffectual.

Fixes: bfd3e9945d ("build: fix x86 out-of-tree build without EFI")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Julien Grall <jgrall@amazon.com>
Release-acked-by: Henry Wang <Henry.Wang@arm.com>

xen/arch/arm/efi/Makefile

@@ -6,6 +6,6 @@ obj-$(CONFIG_ACPI) +=  efi-dom0.init.o
 else
 obj-y += common-stub.o
 
-$(obj)/stub.o: CFLAGS-y += -fno-short-wchar
+$(obj)/common-stub.o: CFLAGS-y += -fno-short-wchar
 
 endif