Add a missing "REQUIRES: system-windows" to a Windows-only test.

This un-breaks builds on other platforms. Otherwise, they fail due to warnings like:

warning: unable to find a Visual Studio installation; try running Clang from a developer command prompt [-Wmsvc-not-found]

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@317716 91177308-0d34-0410-b5e6-96231b3b80d8

test/Driver/coverage.c

@@ -1,4 +1,5 @@
 // Test coverage flag.
+// REQUIRES: system-windows
 //
 // RUN: %clang_cl -### -coverage %s -o foo/bar.o 2>&1 | FileCheck -check-prefix=CLANG-CL-COVERAGE %s
 // CLANG-CL-COVERAGE-NOT: error: