Disable perfschema.mdl_func on Windows

The test fails extremely often on a Windows builder, and nobody seems
to be interested in fixing it.

mysql-test/suite/perfschema/t/mdl_func.test

@@ -1,5 +1,7 @@
 --source include/not_embedded.inc
 --source include/have_perfschema.inc
+# This test fails very frequently on a Windows builder.
+--source include/not_windows.inc
 
 UPDATE performance_schema.setup_instruments SET enabled = 'NO', timed = 'YES';