Revert "remove (empty) SkXfermode.h"

This reverts commit c5619cd1702759990f278ed87c3654a292f044d1.

Reason for revert: Want to keep Google3 green & be able to detect other changes breaking it.

Change-Id: I3a651d0996838ed12bc0cfc6ed464a2cee37fba4
Reviewed-on: https://skia-review.googlesource.com/5381
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
diff --git a/include/core/SkXfermode.h b/include/core/SkXfermode.h
new file mode 100644
index 0000000..199dff2
--- /dev/null
+++ b/include/core/SkXfermode.h
@@ -0,0 +1,13 @@
+/*
+ * Copyright 2006 The Android Open Source Project
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef SkXfermode_DEFINED
+#define SkXfermode_DEFINED
+
+//#error "SkXfermode.h is deprecated"
+
+#endif