Makefile.am: fix adding contrib directory for dist

With extra / the directory was getting added as a subdirectory of
itself.

Change-Id: I5a3d5635f3c949f39d88b34a2ddab8244643f560
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/2070


Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>