[Clang] Disable rewrite-includes-bom.c on Windows

This was failing on
https://lab.llvm.org/buildbot/#/builders/46/builds/25073. This test
seems to have always had portability problems looking at the commit
history. The most recent patch I landed to touch this enabled it on
Windows again, so we are not disabling coverage that was there before.

Author: boomanaiden154

clang/test/Frontend/rewrite-includes-bom.c

@@ -3,5 +3,6 @@
 // RUN: cat %t.c | od -t x1 | not grep -q 'ef\s*bb\s*bf'
 // RUN: %clang_cc1 -fsyntax-only -verify %t.c
 // expected-no-diagnostics
+// UNSUPPORTED: system-windows
 
 #include "rewrite-includes-bom.h"