Relicense to LGPL3

As the students' implementation of the pi2 is not free software,
we have to relicense the pi2-view library to allow linking
with non-free software.

meson.build

 project('pi2-view', ['cpp', 'c'],
-    license: 'GPL3',
+    license: 'LGPL3',
     meson_version: '>=0.45.1',
     version: '0.1',
     default_options : ['warning_level=3', 'cpp_args=-Wno-unknown-pragmas', 'werror=true', 'cpp_std=c++17', 'c_std=c11']