openmotif: update SRC_URI

openmotif-2.3.3 doesn't appear to be available from fossies.org anymore. As this
isn't the official source for openmotif, we change the SRC_URI rather than
adding a mirror.

The official source requires registration to download the archive (via
https://www2.opengroup.org/ogsys/catalog/zom213

) so this is the best we can do.

Signed-off-by: Paul Barker <paul@paulbarker.me.uk>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>