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: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>