[PATCH] powerpc: Fix clean_files in arch/powerpc/boot Makefile

clean-files was being set twice rather than being appended to.
Signed-off-by: NKumar Gala <galak@kernel.crashing.org>
Signed-off-by: NPaul Mackerras <paulus@samba.org>

arch/powerpc/boot/Makefile

@@ -176,4 +176,4 @@ $(obj)/uImage: $(obj)/vmlinux.gz
 install: $(CONFIGURE) $(BOOTIMAGE)
 	sh -x $(srctree)/$(src)/install.sh "$(KERNELRELEASE)" vmlinux System.map "$(INSTALL_PATH)" "$(BOOTIMAGE)"
 
-clean-files := $(addprefix $(objtree)/, $(obj-boot) vmlinux.strip)
+clean-files += $(addprefix $(objtree)/, $(obj-boot) vmlinux.strip)