asm-generic headers: add ftrace.h

This patch adds ftrace.h into asm-generic headers.
The file content could be empty in most architectures.
Signed-off-by: Guan Xuetao <gxt@mprc.pku.edu.cn>
Acked-by: Arnd Bergmann <arnd@arndb.de>

include/asm-generic/ftrace.h

+/*
+ * linux/include/asm-generic/ftrace.h
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ */
+#ifndef __ASM_GENERIC_FTRACE_H__
+#define __ASM_GENERIC_FTRACE_H__
+
+/*
+ * Not all architectures need their own ftrace.h, the most
+ * common definitions are already in linux/ftrace.h.
+ */
+
+#endif /* __ASM_GENERIC_FTRACE_H__ */