kbuild: ensure mrproper removes .old_version

If the final linking of vmlinux fails, the file .old_version are left
behind.  This patch ensures the mrproper target will remove it if
present.
Signed-off-by: Tore Anderson <tore@fud.no>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

Makefile

@@ -984,7 +984,7 @@ CLEAN_FILES +=	vmlinux System.map \
 
 # Directories & files removed with 'make mrproper'
 MRPROPER_DIRS  += include/config include2
-MRPROPER_FILES += .config .config.old include/asm .version \
+MRPROPER_FILES += .config .config.old include/asm .version .old_version \
                   include/linux/autoconf.h include/linux/version.h \
                   Module.symvers tags TAGS cscope*