kbuild: fix make O=... build

It fixes the following error:

make[1]: *** No rule to make target `include/asm', needed by `arch/alpha/kernel/asm-offsets.s'.  Stop.

Reported by:
From: Jan Dittmer <j.dittmer@portrix.net>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

Makefile

@@ -779,7 +779,7 @@ endif
 # prepare1 creates a makefile if using a separate output directory
 prepare1: prepare2 outputmakefile
 
-prepare0: prepare1 include/linux/version.h $(objtree)/include/asm \
+prepare0: prepare1 include/linux/version.h include/asm \
                    include/config/MARKER
 ifneq ($(KBUILD_MODULES),)
 	$(Q)rm -rf $(MODVERDIR)
@@ -819,7 +819,7 @@ export CPPFLAGS_vmlinux.lds += -P -C -U$(ARCH)
 #	hard to detect, but I suppose "make mrproper" is a good idea
 #	before switching between archs anyway.
 
-$(objtree)/include/asm:
+include/asm:
 	@echo '  SYMLINK $@ -> include/asm-$(ARCH)'
 	$(Q)if [ ! -d include ]; then mkdir -p include; fi;
 	@ln -fsn asm-$(ARCH) $@