build: Fix the inclusion paths for GIR files in gi-docgen

The GIR files are now built by GLib itself, so they will be in the build
directories of each sub-library, except for GLib-2.0 which is built
alongside GObject-2.0.

docs/reference/glib/meson.build

@@ -178,6 +178,7 @@ custom_target('glib-docs',
     '--config=@INPUT0@',
     '--output-dir=@OUTPUT@',
     '--content-dir=@0@'.format(meson.current_source_dir()),
+    '--add-include-path=@0@'.format(meson.current_build_dir() / '../../../gobject'),
     '@INPUT1@',
   ],
   build_by_default: true,