x86: Remove duplicated #include

Remove duplicated #include in:

  arch/x86/kernel/dumpstack.c
Signed-off-by: NHuang Weiyi <weiyi.huang@gmail.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

arch/x86/kernel/dumpstack.c

@@ -15,7 +15,6 @@
 #include <linux/bug.h>
 #include <linux/nmi.h>
 #include <linux/sysfs.h>
-#include <linux/ftrace.h>
 
 #include <asm/stacktrace.h>