[NFC] skip the test modules-vtable.cppm on windows

The new added test has problems on windows since the patch is about ABI
and MSVC ABI is not covered. Skip the test on windows to make the CI
green.

Author: ChuanqiXu9

clang/test/CodeGenCXX/modules-vtable.cppm

@@ -1,3 +1,5 @@
+// REQUIRES: !system-windows
+
 // RUN: rm -rf %t
 // RUN: split-file %s %t
 // RUN: cd %t