x11-toolkits/kproperty: add LICENSE

KProperty is a library, which follows the KDE (Frameworks) licensing
policy, so it is LGPLv2.1 or (at your option) any later version.
Parts of the sources (some CMake-related files) are MIT or Boost-
licensed, but those don't get installed.

Reported-by: Chris Hutchinson
PR: 279065

x11-toolkits/kproperty/Makefile

-PORTNAME=		kproperty
-DISTVERSION=		3.2.0
-PORTREVISION=	2
-CATEGORIES=		x11-toolkits kde
-MASTER_SITES=		KDE/stable/${PORTNAME}/src
-DIST_SUBDIR=		KDE/${PORTNAME}
+PORTNAME=	kproperty
+DISTVERSION=	3.2.0
+PORTREVISION=	3
+CATEGORIES=	x11-toolkits kde
+MASTER_SITES=	KDE/stable/${PORTNAME}/src
+DIST_SUBDIR=	KDE/${PORTNAME}
 
-MAINTAINER=		kde@FreeBSD.org
-COMMENT=		Property editing framwork
+MAINTAINER=	kde@FreeBSD.org
+COMMENT=	Property editing framwork
 WWW=		https://www.kexi-project.org/
 
+LICENSE=	LGPL21+
+LICENSE_FILE=	COPYING.LIB
+
 USES=		cmake compiler:c++11-lang gettext kde:5 qt:5 tar:xz
 USE_KDE=	config coreaddons guiaddons i18n widgetsaddons \
 		ecm:build