xen/hypervisor: Don't use cc-option-add for -Wdeclaration-after-statement

Per Andrew's comment in [1] all the compilers we support should
recognize the flag.

I forgot to address the comment while committing.

[1] fcf00090-304a-49f7-8a61-a54347e90a3b@citrix.com

Signed-off-by: Julien Grall <jgrall@amazon.com>

xen/Makefile

@@ -393,7 +393,7 @@ CFLAGS-$(CONFIG_CC_SPLIT_SECTIONS) += -ffunction-sections -fdata-sections
 
 CFLAGS += -nostdinc -fno-builtin -fno-common
 CFLAGS += -Werror -Wredundant-decls -Wwrite-strings -Wno-pointer-arith
-$(call cc-option-add,CFLAGS,CC,-Wdeclaration-after-statement)
+CFLAGS += -Wdeclaration-after-statement
 $(call cc-option-add,CFLAGS,CC,-Wvla)
 CFLAGS += -pipe -D__XEN__ -include $(srctree)/include/xen/config.h
 CFLAGS-$(CONFIG_DEBUG_INFO) += -g