Move kasprintf.o to obj-y

Modulat lguest started giving linking errors

MODPOST 1 modules
ERROR: "kasprintf" [drivers/lguest/lg.ko] undefined!

Signed-off-by: Alexey Dobriyan <adobriyan@sw.ru>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>