build: Make the introspection feature yield

This is expected by other projects who use glib as a subproject,
and there is little point in making the option a feature unless
you inherit it.

meson.options

@@ -148,6 +148,7 @@ option('gir_dir_prefix',
 option('introspection',
        type: 'feature',
        value: 'auto',
+       yield: true,
        description: 'Enable generating introspection data (requires gobject-introspection)')
 
 option('file_monitor_backend',