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>