kbuild: fix installing external modules

Eric Sandeen <sandeen@redhat.com> reported:
  Installing external modules is supposed to put them in some path
  under /lib/modules/<version>/extra/subdir/, but this change:
  http://linux.bkbits.net:8080/linux-2.6/?PAGE=cset&REV=1.1982.9.23


  makes them go under /lib/modules/<version>/extrasubdir

 (for example, make M=fs/ext3 modules_install puts ext3.ko in
 /lib/modules/<version>/extrafs/ext3.ko)

This was the case only when specifying a trailing slash to M=..

Fixed by removing trailing slash if present so
we correctly match dir part of target.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Eric Sandeen <sandeen@redhat.com>