meson: Install libxml2.py

Apparently, nobody has ever tried to use the Python bindings built with
Meson.

python/meson.build

@@ -19,6 +19,8 @@ if py.found() == true
             'libxml2.py',
         ],
         command: [py, '@INPUT@', meson.current_build_dir()],
+        install: true,
+        install_dir: [ false, false, false, py.get_install_dir() ],
     )
     pygenerated_dep = declare_dependency(
         sources : [pygenerated[0]],