When an external module is being built in down in a directory structure
keep the relative directory when installing the module.

Example:
fs/ contains a Makefile used to build both modules:
obj-y := myfs/ oldfs/
Install directories
fs/myfs/myfs.ko	  => Will be installed in /lib/modules/<version>/extra/fs/myfs/
fs/oldfs/oldfs.o  => Will be installed in /lib/modules/<version>/extra/fs/oldfs/
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>