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>

Makefile.am

@@ -31,7 +31,7 @@ EXTRA_DIST = \
 	tools/logger.pl \
 	tools/rlink_make_speed_table \
 	tools/st7_dtc_as \
-	contrib/
+	contrib
 
 libtool: $(LIBTOOL_DEPS)
 	$(SHELL) ./config.status --recheck