x11-toolkits/linux-c7-openmotif: add LICENSE field
Metadata-only change, no PORTREVISION bump needed as this does not affect the built package. Port is DEPRECATED (CentOS 7 EOL) but has no expiration date set. PR: [[ https://bugs.freebsd.org/279068 | 279068 ]] Approved by: emulation@FreeBSD.org (maintainer timeout)