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