meson: set install_tag for schema files

To limit the amount of files installed by meson install, users can specify specific classes of files they actually want to install.

Most to be installed files are automatically tagged by meson correctly based on what function produced them, but it can't for some (esp. those installed using install_data/subdir).

As gschema files should typically be available at runtime, give them the "runtime" tag.

See https://mesonbuild.com/Installing.html#installation-tags