MacOSX: add missing path separator

This fixes https://github.com/imagej/imagej-launcher/issues/21 (i.e.
java.ext.dirs being set erroneously), reported by Kota Miura.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Merge pull request #21 from imagej/mac-j3d

Fix java.ext.dirs path on OS X