Add license meta-data explicitly

Avoid warning from c++utilities' CMake module and have it
ready for an AppStream meta-data file.

src/CMakeLists.txt

@@ -5,6 +5,7 @@ cmake_minimum_required(VERSION 3.1.0 FATAL_ERROR)
 project(pianobooster)
 set(META_PROJECT_NAME ${PROJECT_NAME})
 set(META_PROJECT_TYPE application)
+set(META_PROJECT_LICENSE "GPL-3.0-or-later")
 set(META_APP_NAME "Piano Booster")
 set(META_APP_CATEGORIES "Music;Education;")
 set(META_APP_AUTHOR "Martchus")