xtensa: remove unnecessary KBUILD_SRC ifeq conditional

You can always prefix variant/platform header search paths with
$(srctree)/ because $(srctree) is '.' for in-tree building.
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: NMax Filippov <jcmvbkbc@gmail.com>

arch/xtensa/Makefile

@@ -64,11 +64,7 @@ endif
 vardirs := $(patsubst %,arch/xtensa/variants/%/,$(variant-y))
 plfdirs := $(patsubst %,arch/xtensa/platforms/%/,$(platform-y))
 
-ifeq ($(KBUILD_SRC),)
-KBUILD_CPPFLAGS += $(patsubst %,-I%include,$(vardirs) $(plfdirs))
-else
 KBUILD_CPPFLAGS += $(patsubst %,-I$(srctree)/%include,$(vardirs) $(plfdirs))
-endif
 
 KBUILD_DEFCONFIG := iss_defconfig