Fix mesonintrospect --installed

The prefix has been recently added to the install data

mesonbuild/mintro.py

@@ -62,7 +62,7 @@ def determine_installed_path(target, installdata):
 def list_installed(installdata):
     res = {}
     if installdata is not None:
-        for path, installpath in installdata.data:
+        for path, installpath, unused_prefix in installdata.data:
             res[path] = os.path.join(installdata.prefix, installpath)
     print(json.dumps(res))