kbuild: Fix build as root then user

From: Matthew Wilcox <matthew@wil.cx>
I inadvertently built a tree as root and then rebuilt it as a user.  I
got a lot of prompts ...

mv: overwrite `drivers/char/drm/drm_auth.o', overriding mode 0644?

Using mv -f fixes that.
Signed-off-by: Matthew Wilcox <matthew@wil.cx>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

scripts/Makefile.build

@@ -169,7 +169,7 @@ cmd_modversions =							\
 			-T $(@D)/.tmp_$(@F:.o=.ver);			\
 		rm -f $(@D)/.tmp_$(@F) $(@D)/.tmp_$(@F:.o=.ver);	\
 	else								\
-		mv $(@D)/.tmp_$(@F) $@;					\
+		mv -f $(@D)/.tmp_$(@F) $@;				\
 	fi;
 endif