build: Tell gi-docgen where to find the GIR files

Now that all files are located under top_srcdir/introspection, we need
to ensure that gi-docgen can find them.

docs/reference/gobject/meson.build

@@ -41,7 +41,6 @@ custom_target('gobject-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,