[PATCH] Fix building external modules on ppc32

We are setting up sources for building external modules like this:

  /usr/src/linux-obj> # create a .config file
  /usr/src/linux-obj> make -C /usr/src/linux O=$PWD oldconfig
  /usr/src/linux-obj> make -C /usr/src/linux O=$PWD prepare
  /usr/src/linux-obj> make -C /usr/src/linux O=$PWD scripts
  /usr/src/linux-obj> make -C /usr/src/linux O=$PWD clean

After that, external modules can be built with:

  /usr/src/module> make -C /usr/src/linux-obj M=$PWD

This fails for ppc32 because the `make clean' removes the
arch/powerpc/include directory.  This should be done in archmrproper
instead of in archclean.

Signed-off-by: Andreas Gruenbacher <agruen@suse.de>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>