adjust x86 EFI build

While the rule to generate .init.o files from .o ones already correctly
included $(extra-y), the setting of the necessary compiler flag didn't
have the same. With some yet to be posted patch this resulted in build
breakage because of the compiler deciding not to inline a few functions
(which then results in .text not being empty as required for these
object files).

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/Rules.mk
+++ b/xen/Rules.mk
@@ -101,7 +101,7 @@ obj-y    := $(patsubst %/,%/built-in.o,$
 
 subdir-all := $(subdir-y) $(subdir-n)
 
-$(filter %.init.o,$(obj-y) $(obj-bin-y)): CFLAGS += -DINIT_SECTIONS_ONLY
+$(filter %.init.o,$(obj-y) $(obj-bin-y) $(extra-y)): CFLAGS += -DINIT_SECTIONS_ONLY
 
 $(obj-$(coverage)): CFLAGS += -fprofile-arcs -ftest-coverage -DTEST_COVERAGE