build: If GLib is a subproject, disable GObject-Introspection

We don't need this and it will only slow down the build.

Signed-off-by: Simon McVittie <smcv@collabora.com>

meson.build

@@ -386,6 +386,7 @@ else
         required: get_option('modular_tests'),
         fallback: ['glib', 'libglib_dep'],
         default_options: fallback_subproject_options + [
+            'introspection=disabled',
             'tests=false',
         ],
     )