[clang] Mark clang-format-ignore.cpp as unsupported on Windows

To heal bots that have been broken for days while discussions on
llvm#76733 are ongoing.

Author: nico

clang/test/Format/clang-format-ignore.cpp

@@ -1,3 +1,4 @@
+// UNSUPPORTED: system-windows
 // RUN: rm -rf %t.dir
 // RUN: mkdir -p %t.dir/level1/level2