kbuild: Do not run make clean in $(srctree)

Signed-off-by: Michal Marek <mmarek@suse.cz>

Makefile

@@ -1180,7 +1180,7 @@ MRPROPER_FILES += .config .config.old .version .old_version             \
 #
 clean: rm-dirs  := $(CLEAN_DIRS)
 clean: rm-files := $(CLEAN_FILES)
-clean-dirs      := $(addprefix _clean_,$(srctree) $(vmlinux-alldirs) Documentation)
+clean-dirs      := $(addprefix _clean_, . $(vmlinux-alldirs) Documentation)
 
 PHONY += $(clean-dirs) clean archclean
 $(clean-dirs):